diff --git a/.pql/pql-plan.json b/.pql/pql-plan.json
index 71720fe5..58f53a07 100644
--- a/.pql/pql-plan.json
+++ b/.pql/pql-plan.json
@@ -1,5 +1,5 @@
 {
-  "exported_at": "2026-05-06T21:03:21Z",
+  "exported_at": "2026-05-06T21:13:42Z",
   "decisions": [
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@@ -9,7 +9,7 @@
       "status": "active",
       "date": "2026-04-20",
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+      "synced_at": "2026-05-06 21:12:28"
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@@ -19,7 +19,7 @@
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       "date": "2026-04-20",
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+      "synced_at": "2026-05-06 21:12:28"
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@@ -29,7 +29,7 @@
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       "date": "2026-04-20",
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@@ -39,7 +39,7 @@
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       "date": "2026-04-20",
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@@ -49,7 +49,7 @@
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@@ -59,7 +59,7 @@
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@@ -69,7 +69,7 @@
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@@ -79,7 +79,7 @@
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@@ -89,7 +89,7 @@
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@@ -99,7 +99,7 @@
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@@ -109,7 +109,7 @@
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@@ -119,7 +119,7 @@
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@@ -129,7 +129,7 @@
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@@ -139,7 +139,7 @@
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+      "synced_at": "2026-05-06 21:12:28"
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@@ -149,7 +149,7 @@
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@@ -159,7 +159,7 @@
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@@ -169,7 +169,7 @@
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@@ -179,7 +179,7 @@
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@@ -189,7 +189,7 @@
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@@ -199,7 +199,7 @@
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@@ -209,7 +209,7 @@
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@@ -219,7 +219,7 @@
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@@ -229,7 +229,7 @@
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@@ -239,7 +239,7 @@
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@@ -249,7 +249,7 @@
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       "date": "2026-04-21",
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+      "synced_at": "2026-05-06 21:12:28"
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@@ -259,7 +259,7 @@
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+      "synced_at": "2026-05-06 21:12:28"
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@@ -269,7 +269,7 @@
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@@ -279,7 +279,7 @@
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@@ -289,7 +289,7 @@
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@@ -299,7 +299,7 @@
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@@ -309,7 +309,7 @@
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@@ -319,7 +319,7 @@
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@@ -329,7 +329,7 @@
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+      "synced_at": "2026-05-06 21:12:28"
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@@ -339,7 +339,7 @@
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+      "synced_at": "2026-05-06 21:12:28"
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@@ -349,7 +349,7 @@
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+      "synced_at": "2026-05-06 21:12:28"
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@@ -359,7 +359,7 @@
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+      "synced_at": "2026-05-06 21:12:28"
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@@ -369,7 +369,7 @@
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@@ -379,7 +379,7 @@
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@@ -389,7 +389,7 @@
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@@ -399,7 +399,7 @@
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+      "synced_at": "2026-05-06 21:12:28"
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@@ -409,7 +409,7 @@
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       "date": "2026-04-22",
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@@ -419,7 +419,7 @@
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@@ -429,7 +429,7 @@
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@@ -439,7 +439,7 @@
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@@ -449,7 +449,7 @@
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       "file_path": "decisions/extensions.md",
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+      "synced_at": "2026-05-06 21:12:28"
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@@ -459,7 +459,7 @@
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@@ -469,7 +469,7 @@
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@@ -479,7 +479,7 @@
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@@ -489,7 +489,7 @@
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@@ -499,7 +499,7 @@
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@@ -509,7 +509,7 @@
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@@ -519,7 +519,7 @@
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@@ -529,7 +529,7 @@
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@@ -539,7 +539,7 @@
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@@ -549,7 +549,7 @@
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@@ -559,7 +559,7 @@
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@@ -569,7 +569,7 @@
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@@ -579,7 +579,7 @@
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@@ -589,7 +589,7 @@
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@@ -599,7 +599,7 @@
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@@ -609,7 +609,7 @@
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@@ -619,7 +619,7 @@
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@@ -629,7 +629,7 @@
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@@ -639,7 +639,7 @@
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@@ -649,7 +649,7 @@
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@@ -658,7 +658,7 @@
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-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
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@@ -667,7 +667,7 @@
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-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
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@@ -676,7 +676,7 @@
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-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
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@@ -685,7 +685,7 @@
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       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-5",
@@ -694,7 +694,7 @@
       "title": "IPC wire-format stability + `schema_version:`",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-6",
@@ -703,7 +703,7 @@
       "title": "Window chrome — native frame vs frameless custom",
       "status": "resolved",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-7",
@@ -712,7 +712,7 @@
       "title": "macOS app bundle signing / notarisation",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-8",
@@ -721,7 +721,7 @@
       "title": "Extension API shape — widgets, subcommands, both?",
       "status": "open",
       "file_path": "decisions/questions-extensions.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-9",
@@ -730,7 +730,7 @@
       "title": "Lua runtime vendoring",
       "status": "open",
       "file_path": "decisions/questions-extensions.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-10",
@@ -739,7 +739,7 @@
       "title": "Extension manifest `schema_version:`",
       "status": "open",
       "file_path": "decisions/questions-extensions.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-11",
@@ -748,7 +748,7 @@
       "title": "Coverage gates — hard thresholds vs soft reporting",
       "status": "open",
       "file_path": "decisions/questions-testing.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-12",
@@ -757,7 +757,7 @@
       "title": "Screen-reader automation (axe-core via Playwright)",
       "status": "open",
       "file_path": "decisions/questions-testing.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-13",
@@ -766,7 +766,7 @@
       "title": "Web production-mode a11y",
       "status": "open",
       "file_path": "decisions/questions-accessibility.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-14",
@@ -775,7 +775,7 @@
       "title": "i18n plurals / gender / date-format tooling",
       "status": "open",
       "file_path": "decisions/questions-accessibility.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-15",
@@ -784,7 +784,7 @@
       "title": "Editor tab — full LSP vs tree-sitter-only highlight",
       "status": "open",
       "file_path": "decisions/questions-process.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-16",
@@ -793,7 +793,7 @@
       "title": "`tree-sitter-dart` grammar maintenance",
       "status": "open",
       "file_path": "decisions/questions-process.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-17",
@@ -802,7 +802,7 @@
       "title": "Icon set growth",
       "status": "open",
       "file_path": "decisions/questions-process.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-18",
@@ -811,7 +811,7 @@
       "title": "Theme hot-reload in release builds",
       "status": "open",
       "file_path": "decisions/questions-process.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-19",
@@ -820,7 +820,7 @@
       "title": "(withdrawn)",
       "status": "resolved",
       "file_path": "decisions/questions-process.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-20",
@@ -829,7 +829,7 @@
       "title": "Kernel DB service — namespaced SQL access?",
       "status": "open",
       "file_path": "decisions/questions-process.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-21",
@@ -838,7 +838,7 @@
       "title": "Pql absorbs planning vs keeps separate",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-22",
@@ -847,7 +847,7 @@
       "title": "Ticket persistence strategy",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-23",
@@ -856,7 +856,7 @@
       "title": "SSH-remote development — run clide against a remote workspace",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-25",
@@ -865,7 +865,7 @@
       "title": "Body text face — mono everywhere vs Josefin Sans UI + mono code",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-26",
@@ -874,7 +874,7 @@
       "title": "Small screen layout (\u003c 1000px)",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-27",
@@ -883,7 +883,7 @@
       "title": "Two-editor split",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-28",
@@ -892,7 +892,7 @@
       "title": "Terminal strip scope — shell only or logs/errors/tests",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-29",
@@ -901,7 +901,7 @@
       "title": "Branch picker location",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-30",
@@ -910,7 +910,7 @@
       "title": "Focus behavior when editor is dirty and viewer is peeked",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "Q-31",
@@ -919,7 +919,7 @@
       "title": "XWayland fallback for frameless — proper Wayland protocol needed",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "R-2",
@@ -929,7 +929,7 @@
       "status": "active",
       "date": "2026-04-20",
       "file_path": "decisions/rejected.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "R-3",
@@ -939,7 +939,7 @@
       "status": "active",
       "date": "2026-04-21",
       "file_path": "decisions/rejected.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "R-4",
@@ -949,7 +949,7 @@
       "status": "active",
       "date": "2026-04-21",
       "file_path": "decisions/rejected.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "R-5",
@@ -959,7 +959,7 @@
       "status": "active",
       "date": "2026-04-21",
       "file_path": "decisions/rejected.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "R-6",
@@ -969,7 +969,7 @@
       "status": "active",
       "date": "2026-04-21",
       "file_path": "decisions/rejected.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "R-7",
@@ -979,7 +979,7 @@
       "status": "active",
       "date": "2026-04-21",
       "file_path": "decisions/rejected.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "R-8",
@@ -989,7 +989,7 @@
       "status": "active",
       "date": "2026-04-21",
       "file_path": "decisions/rejected.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "R-9",
@@ -999,7 +999,7 @@
       "status": "active",
       "date": "2026-04-21",
       "file_path": "decisions/rejected.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "R-10",
@@ -1009,7 +1009,7 @@
       "status": "active",
       "date": "2026-04-21",
       "file_path": "decisions/rejected.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "R-11",
@@ -1019,7 +1019,7 @@
       "status": "active",
       "date": "2026-04-21",
       "file_path": "decisions/rejected.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     },
     {
       "id": "R-12",
@@ -1029,7 +1029,7 @@
       "status": "active",
       "date": "2026-04-22",
       "file_path": "decisions/rejected.md",
-      "synced_at": "2026-05-06 21:00:50"
+      "synced_at": "2026-05-06 21:12:28"
     }
   ],
   "decision_refs": [
diff --git a/pubspec.yaml b/pubspec.yaml
index 7edf2e40..f1634a60 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -18,7 +18,7 @@ repository: https://github.com/postmeridiem/clide
 
 # Pre-push line-coverage floor. Ratchets up only — see D-66.
 # Reading: `awk -F: '/^coverage_floor:/ {gsub(/ /,"",$2); print $2}' pubspec.yaml`.
-coverage_floor: 39
+coverage_floor: 43
 
 # Project metadata (was project.yaml, folded in per D-056).
 # version: above is the single source of truth. The Makefile reads
diff --git a/test/terminal/buffer/buffer_test.dart b/test/terminal/buffer/buffer_test.dart
new file mode 100644
index 00000000..86da50c4
--- /dev/null
+++ b/test/terminal/buffer/buffer_test.dart
@@ -0,0 +1,802 @@
+/// Pure-Dart tests for Buffer (lib/src/terminal/src/core/buffer/buffer.dart).
+///
+/// Buffer is the orchestrator on top of BufferLine — viewport sizing,
+/// scroll regions, cursor movement, erase commands, scroll/index/reverse-
+/// index, line insert/delete, resize + reflow, and word-boundary lookup.
+library;
+
+import 'package:clide/src/terminal/src/core/buffer/buffer.dart';
+import 'package:clide/src/terminal/src/core/buffer/cell_offset.dart';
+import 'package:clide/src/terminal/src/core/buffer/range_line.dart';
+import 'package:clide/src/terminal/src/core/cursor.dart';
+import 'package:clide/src/terminal/src/core/mouse/mode.dart';
+import 'package:clide/src/terminal/src/core/state.dart';
+import 'package:test/test.dart';
+
+class _State implements TerminalState {
+  _State({
+    this.viewWidth = 10,
+    this.viewHeight = 5,
+    this.autoWrapMode = true,
+    this.lineFeedMode = false,
+    this.originMode = false,
+    this.reflowEnabled = false,
+  });
+
+  @override
+  int viewWidth;
+  @override
+  int viewHeight;
+  @override
+  bool autoWrapMode;
+  @override
+  bool lineFeedMode;
+  @override
+  bool originMode;
+  @override
+  bool reflowEnabled;
+
+  @override
+  final cursor = CursorStyle();
+
+  // Unused-by-Buffer bits — dummy values are fine.
+  @override
+  bool get insertMode => false;
+  @override
+  bool get cursorKeysMode => false;
+  @override
+  bool get reverseDisplayMode => false;
+  @override
+  MouseMode get mouseMode => MouseMode.none;
+  @override
+  MouseReportMode get mouseReportMode => MouseReportMode.normal;
+  @override
+  bool get cursorBlinkMode => true;
+  @override
+  bool get cursorVisibleMode => true;
+  @override
+  bool get appKeypadMode => false;
+  @override
+  bool get reportFocusMode => false;
+  @override
+  bool get altBufferMouseScrollMode => false;
+  @override
+  bool get bracketedPasteMode => false;
+}
+
+Buffer _newBuffer({
+  int viewWidth = 10,
+  int viewHeight = 5,
+  int maxLines = 50,
+  bool isAltBuffer = false,
+  bool autoWrapMode = true,
+  bool lineFeedMode = false,
+  bool originMode = false,
+  bool reflowEnabled = false,
+  Set<int>? wordSeparators,
+}) {
+  final state = _State(
+    viewWidth: viewWidth,
+    viewHeight: viewHeight,
+    autoWrapMode: autoWrapMode,
+    lineFeedMode: lineFeedMode,
+    originMode: originMode,
+    reflowEnabled: reflowEnabled,
+  );
+  return Buffer(
+    state,
+    maxLines: maxLines,
+    isAltBuffer: isAltBuffer,
+    wordSeparators: wordSeparators,
+  );
+}
+
+void main() {
+  group('Buffer — construction', () {
+    test('seeds viewHeight empty lines and resets margins', () {
+      final b = _newBuffer(viewWidth: 8, viewHeight: 4);
+      expect(b.height, 4);
+      expect(b.viewWidth, 8);
+      expect(b.viewHeight, 4);
+      expect(b.cursorX, 0);
+      expect(b.cursorY, 0);
+      expect(b.scrollBack, 0);
+      expect(b.marginTop, 0);
+      expect(b.marginBottom, 3);
+    });
+
+    test('absolute cursor + margin getters are scrollback-shifted', () {
+      final b = _newBuffer(viewHeight: 3);
+      // Push a line via index() in the no-scrollable-region path on a
+      // primary buffer — that uses lines.push to grow scrollback.
+      b.setCursor(0, 2);
+      b.index();
+      // Now scrollBack == 1; the cursor stayed at viewHeight - 1 = 2.
+      expect(b.scrollBack, 1);
+      expect(b.absoluteCursorY, 3);
+      expect(b.absoluteMarginTop, 1);
+      expect(b.absoluteMarginBottom, 3);
+    });
+  });
+
+  group('Buffer — cursor movement and clamping', () {
+    test('setCursorX/Y clamp to viewWidth-1 / viewHeight-1', () {
+      final b = _newBuffer(viewWidth: 5, viewHeight: 3);
+      b.setCursorX(99);
+      b.setCursorY(99);
+      expect(b.cursorX, 4);
+      expect(b.cursorY, 2);
+      b.setCursorX(-1);
+      b.setCursorY(-1);
+      expect(b.cursorX, 0);
+      expect(b.cursorY, 0);
+    });
+
+    test('moveCursorX / moveCursorY are relative + clamped', () {
+      final b = _newBuffer();
+      b.setCursor(5, 2);
+      b.moveCursorX(2);
+      b.moveCursorY(1);
+      expect(b.cursorX, 7);
+      expect(b.cursorY, 3);
+      b.moveCursorX(99); // clamps
+      expect(b.cursorX, 9);
+    });
+
+    test('cursorGoForward saturates at viewWidth (one past last visible)', () {
+      final b = _newBuffer(viewWidth: 5);
+      for (var i = 0; i < 10; i++) {
+        b.cursorGoForward();
+      }
+      // The clamped getter caps at viewWidth - 1; the internal saturation
+      // sits at viewWidth.
+      expect(b.cursorX, 4);
+    });
+
+    test('setCursor — originMode shifts y by marginTop and clamps to marginBottom', () {
+      final b = _newBuffer(viewHeight: 6, originMode: true);
+      b.setVerticalMargins(2, 4);
+      b.setCursor(0, 0);
+      // origin shift: cursorY += marginTop (2) → clamped to marginBottom (4).
+      expect(b.cursorY, 2);
+      b.setCursor(0, 99);
+      expect(b.cursorY, 4); // clamped to marginBottom
+    });
+
+    test('moveCursor delegates to setCursor', () {
+      final b = _newBuffer();
+      b.setCursor(0, 0);
+      b.moveCursor(2, 1);
+      expect(b.cursorX, 2);
+      expect(b.cursorY, 1);
+    });
+  });
+
+  group('Buffer — vertical margins', () {
+    test('setVerticalMargins clamps and orders top/bottom', () {
+      final b = _newBuffer(viewHeight: 5);
+      b.setVerticalMargins(99, -5);
+      // Both clamp to [0, viewHeight-1] = [0, 4]; the implementation
+      // also coerces top<=bottom by min/max swap.
+      expect(b.marginTop, lessThanOrEqualTo(b.marginBottom));
+    });
+
+    test('resetVerticalMargins restores full viewport range', () {
+      final b = _newBuffer(viewHeight: 5);
+      b.setVerticalMargins(1, 3);
+      b.resetVerticalMargins();
+      expect(b.marginTop, 0);
+      expect(b.marginBottom, 4);
+    });
+
+    test('isInVerticalMargin is true inside, false outside', () {
+      final b = _newBuffer(viewHeight: 5);
+      b.setVerticalMargins(1, 3);
+      b.setCursorY(0);
+      expect(b.isInVerticalMargin, isFalse);
+      b.setCursorY(2);
+      expect(b.isInVerticalMargin, isTrue);
+      b.setCursorY(4);
+      expect(b.isInVerticalMargin, isFalse);
+    });
+  });
+
+  group('Buffer — write / writeChar / autoWrap', () {
+    test('write adds chars left-to-right and advances the cursor', () {
+      final b = _newBuffer(viewWidth: 5);
+      b.write('abc');
+      expect(b.cursorX, 3);
+      expect(b.currentLine.getCodePoint(0), 'a'.codeUnitAt(0));
+      expect(b.currentLine.getCodePoint(1), 'b'.codeUnitAt(0));
+      expect(b.currentLine.getCodePoint(2), 'c'.codeUnitAt(0));
+    });
+
+    test('autoWrap pushes the cursor to the next line and marks it wrapped', () {
+      final b = _newBuffer(viewWidth: 3, viewHeight: 4);
+      b.write('abcdef'); // 'def' wraps to line 1
+      expect(b.height, 4); // still within view; no scrollback needed yet
+      expect(b.cursorY, 1);
+      // cursorX getter clamps to viewWidth-1; internal _cursorX is saturated.
+      expect(b.cursorX, 2);
+      expect(b.currentLine.isWrapped, isTrue);
+    });
+
+    test('autoWrap=false leaves the cursor saturated past the end (no wrap)', () {
+      final b = _newBuffer(viewWidth: 3, autoWrapMode: false);
+      b.write('abc');
+      // After three writes the cursor is at viewWidth (3). The next write
+      // triggers index() but autoWrapMode is off — the line shouldn't be
+      // marked wrapped.
+      b.write('d');
+      expect(b.currentLine.isWrapped, isFalse);
+    });
+
+    test('wide character writes a width-2 cell + a 0-codepoint trailer', () {
+      final b = _newBuffer(viewWidth: 5);
+      b.write('中'); // CJK ideograph, width 2
+      expect(b.cursorX, 2);
+    });
+  });
+
+  group('Buffer — backspace', () {
+    test('column 0 on a wrapped line jumps to the prior line end and unwraps', () {
+      final b = _newBuffer(viewWidth: 3, viewHeight: 3);
+      b.write('abcd'); // wraps; cursor at (1, 1) on a wrapped line
+      expect(b.currentLine.isWrapped, isTrue);
+      b.setCursor(0, 1);
+      // Re-flag wrapped (setCursor may not affect it but to be sure)
+      b.currentLine.isWrapped = true;
+      b.backspace();
+      expect(b.cursorX, 2); // viewWidth - 1
+      expect(b.cursorY, 0);
+    });
+
+    test('column == viewWidth backspaces by 2 cells (off the saturated edge)', () {
+      final b = _newBuffer(viewWidth: 5);
+      // Saturate the cursor.
+      for (var i = 0; i < 5; i++) {
+        b.cursorGoForward();
+      }
+      // Internal _cursorX is now 5 (viewWidth). The clamped getter shows 4.
+      b.backspace();
+      // -2 of (5) clamps to (3). cursorX getter returns clamped value.
+      expect(b.cursorX, 3);
+    });
+
+    test('mid-line backspace is just a -1', () {
+      final b = _newBuffer(viewWidth: 5);
+      b.setCursor(3, 0);
+      b.backspace();
+      expect(b.cursorX, 2);
+    });
+  });
+
+  group('Buffer — erase commands', () {
+    test('eraseLine clears the whole current line and clears wrapped flag', () {
+      final b = _newBuffer(viewWidth: 5);
+      b.write('abc');
+      b.currentLine.isWrapped = true;
+      b.eraseLine();
+      expect(b.currentLine.isWrapped, isFalse);
+      for (var i = 0; i < 5; i++) {
+        expect(b.currentLine.getCodePoint(i), 0);
+      }
+    });
+
+    test('eraseLineFromCursor only clears [cursor, viewWidth)', () {
+      final b = _newBuffer(viewWidth: 5);
+      b.write('abcde');
+      b.setCursorX(2);
+      b.eraseLineFromCursor();
+      expect(b.currentLine.getCodePoint(0), 'a'.codeUnitAt(0));
+      expect(b.currentLine.getCodePoint(1), 'b'.codeUnitAt(0));
+      expect(b.currentLine.getCodePoint(2), 0);
+      expect(b.currentLine.getCodePoint(4), 0);
+    });
+
+    test('eraseLineToCursor only clears [0, cursor)', () {
+      final b = _newBuffer(viewWidth: 5);
+      b.write('abcde');
+      b.setCursorX(3);
+      b.eraseLineToCursor();
+      expect(b.currentLine.getCodePoint(0), 0);
+      expect(b.currentLine.getCodePoint(2), 0);
+      expect(b.currentLine.getCodePoint(3), 'd'.codeUnitAt(0));
+    });
+
+    test('eraseChars erases [cursor, cursor+count)', () {
+      final b = _newBuffer(viewWidth: 5);
+      b.write('abcde');
+      b.setCursorX(1);
+      b.eraseChars(2);
+      expect(b.currentLine.getCodePoint(0), 'a'.codeUnitAt(0));
+      expect(b.currentLine.getCodePoint(1), 0);
+      expect(b.currentLine.getCodePoint(2), 0);
+      expect(b.currentLine.getCodePoint(3), 'd'.codeUnitAt(0));
+    });
+
+    test('eraseDisplay clears every visible line', () {
+      final b = _newBuffer(viewWidth: 5, viewHeight: 3);
+      b.write('aaaaa');
+      b.lineFeed();
+      b.write('bbbbb');
+      b.eraseDisplay();
+      for (var y = 0; y < b.viewHeight; y++) {
+        for (var x = 0; x < b.viewWidth; x++) {
+          expect(b.lines[y].getCodePoint(x), 0);
+        }
+      }
+    });
+
+    test('eraseDisplayFromCursor clears current line tail + every line below', () {
+      // lineFeedMode: true so lineFeed resets cursorX — otherwise the
+      // saturated cursorX would make the next write wrap onto an extra
+      // line via writeChar's autoWrap path.
+      final b = _newBuffer(viewWidth: 5, viewHeight: 5, lineFeedMode: true);
+      b.write('aaaaa');
+      b.lineFeed();
+      b.write('bbbbb');
+      b.lineFeed();
+      b.write('ccccc');
+      b.setCursor(2, 1);
+      b.eraseDisplayFromCursor();
+      expect(b.lines[0].getCodePoint(0), 'a'.codeUnitAt(0)); // untouched
+      expect(b.lines[1].getCodePoint(0), 'b'.codeUnitAt(0)); // head kept
+      expect(b.lines[1].getCodePoint(2), 0); // tail erased
+      expect(b.lines[2].getCodePoint(0), 0); // line below wiped
+    });
+
+    test('eraseDisplayToCursor clears every line above + the current line head', () {
+      final b = _newBuffer(viewWidth: 5, viewHeight: 5, lineFeedMode: true);
+      b.write('aaaaa');
+      b.lineFeed();
+      b.write('bbbbb');
+      b.lineFeed();
+      b.write('vwxyz'); // distinct chars so the kept tail is visible
+      b.setCursor(3, 2);
+      b.eraseDisplayToCursor();
+      expect(b.lines[0].getCodePoint(0), 0);
+      expect(b.lines[1].getCodePoint(0), 0);
+      expect(b.lines[2].getCodePoint(0), 0); // head erased
+      expect(b.lines[2].getCodePoint(3), 'y'.codeUnitAt(0)); // tail kept
+      expect(b.lines[2].getCodePoint(4), 'z'.codeUnitAt(0));
+    });
+  });
+
+  group('Buffer — scroll / index / lineFeed / reverseIndex', () {
+    test('scrollUp inside the scroll region rotates lines and seeds an empty tail', () {
+      final b = _newBuffer(viewWidth: 3, viewHeight: 5, lineFeedMode: true);
+      b.write('aaa');
+      b.lineFeed();
+      b.write('bbb');
+      b.lineFeed();
+      b.write('ccc');
+      b.scrollUp(1);
+      // After scroll: line 0 = 'bbb', line 1 = 'ccc', line 2 = empty.
+      expect(b.lines[0].getCodePoint(0), 'b'.codeUnitAt(0));
+      expect(b.lines[1].getCodePoint(0), 'c'.codeUnitAt(0));
+      expect(b.lines[2].getCodePoint(0), 0);
+    });
+
+    test('scrollDown inside the scroll region rotates lines and seeds an empty head', () {
+      final b = _newBuffer(viewWidth: 3, viewHeight: 5, lineFeedMode: true);
+      b.write('aaa');
+      b.lineFeed();
+      b.write('bbb');
+      b.scrollDown(1);
+      // After scroll: line 0 = empty, line 1 = 'aaa', line 2 = 'bbb'.
+      expect(b.lines[0].getCodePoint(0), 0);
+      expect(b.lines[1].getCodePoint(0), 'a'.codeUnitAt(0));
+      expect(b.lines[2].getCodePoint(0), 'b'.codeUnitAt(0));
+    });
+
+    test('index — at the bottom of primary buffer pushes a new line to grow scrollback', () {
+      final b = _newBuffer(viewWidth: 3, viewHeight: 3);
+      b.setCursorY(2);
+      b.index();
+      expect(b.scrollBack, 1);
+      expect(b.cursorY, 2); // stayed at viewHeight - 1
+    });
+
+    test('index — at the bottom of alt buffer scrolls up instead of growing', () {
+      final b = _newBuffer(viewWidth: 3, viewHeight: 3, isAltBuffer: true);
+      b.setCursorY(2);
+      b.index();
+      expect(b.scrollBack, 0);
+      expect(b.cursorY, 2);
+    });
+
+    test('index — within margin but not at the bottom moves cursor down', () {
+      final b = _newBuffer(viewHeight: 5);
+      b.setVerticalMargins(1, 3);
+      b.setCursorY(1);
+      b.index();
+      expect(b.cursorY, 2);
+    });
+
+    test('index — at margin bottom with non-zero top scrollUps inside the region', () {
+      final b = _newBuffer(viewWidth: 3, viewHeight: 4);
+      b.setVerticalMargins(1, 2);
+      b.setCursorY(2); // at marginBottom
+      b.write('a'); // mark line 2
+      b.index();
+      expect(b.cursorY, 2);
+    });
+
+    test('index — at marginBottom with marginTop=0 on primary inserts a new line', () {
+      final b = _newBuffer(viewWidth: 3, viewHeight: 4);
+      // marginTop=0 by default
+      b.setCursorY(b.marginBottom);
+      b.write('z');
+      final beforeHeight = b.height;
+      b.index();
+      expect(b.height, beforeHeight + 1); // an empty line was inserted
+    });
+
+    test('index — cursor outside the vertical margin and at viewport bottom on primary pushes scrollback', () {
+      final b = _newBuffer(viewWidth: 3, viewHeight: 5);
+      // Margins shrink the scroll region — the cursor below it is outside.
+      b.setVerticalMargins(1, 3);
+      b.setCursorY(4); // viewHeight - 1, outside [1, 3]
+      final before = b.height;
+      b.index();
+      expect(b.height, before + 1); // primary path: lines.push
+    });
+
+    test('index — cursor outside the vertical margin on alt buffer scrolls up instead of pushing', () {
+      final b = _newBuffer(viewWidth: 3, viewHeight: 5, isAltBuffer: true);
+      b.setVerticalMargins(1, 3);
+      b.setCursorY(4);
+      final before = b.height;
+      b.index();
+      expect(b.height, before); // alt path: no growth
+    });
+
+    test('index — cursor outside the vertical margin and not at the bottom moves cursor down', () {
+      final b = _newBuffer(viewHeight: 5);
+      b.setVerticalMargins(1, 3);
+      b.setCursorY(4); // outside, but not viewHeight-1... wait viewHeight-1 IS 4
+      // To exercise the moveCursorY branch outside the margin we need
+      // _cursorY < viewHeight - 1 AND outside [marginTop, marginBottom].
+      // With margins (1, 2) the cursor at y=3 is outside and not at the bottom.
+      b.setVerticalMargins(1, 2);
+      b.setCursorY(3);
+      b.index();
+      expect(b.cursorY, 4);
+    });
+
+    test('lineFeed honours lineFeedMode by setting cursor to column 0', () {
+      final b = _newBuffer(lineFeedMode: true);
+      b.setCursorX(5);
+      b.lineFeed();
+      expect(b.cursorX, 0);
+    });
+
+    test('lineFeed leaves cursorX alone when lineFeedMode is off', () {
+      final b = _newBuffer();
+      b.setCursorX(5);
+      b.lineFeed();
+      expect(b.cursorX, 5);
+    });
+
+    test('reverseIndex inside margins at top scrolls down, otherwise moves up', () {
+      final b = _newBuffer(viewHeight: 5);
+      b.setVerticalMargins(1, 3);
+      b.setCursorY(2);
+      b.reverseIndex();
+      expect(b.cursorY, 1);
+
+      b.setCursorY(1); // at marginTop
+      b.reverseIndex();
+      expect(b.cursorY, 1); // cursor stays; scrollDown happened
+
+      b.setCursorY(0); // outside margins
+      b.reverseIndex();
+      expect(b.cursorY, 0); // already at 0, clamps
+    });
+  });
+
+  group('Buffer — saveCursor / restoreCursor', () {
+    test('round-trips position + style + charset', () {
+      final b = _newBuffer();
+      b.setCursor(3, 2);
+      // Mutate cursor style.
+      // (Reach into the underlying state via the property surface.)
+      b.saveCursor();
+      b.setCursor(0, 0);
+      b.restoreCursor();
+      expect(b.cursorX, 3);
+      expect(b.cursorY, 2);
+    });
+  });
+
+  group('Buffer — line insert / delete / chars', () {
+    test('insertBlankChars opens a gap at the cursor', () {
+      final b = _newBuffer(viewWidth: 5);
+      b.write('abcde');
+      b.setCursorX(1);
+      b.insertBlankChars(2);
+      expect(b.currentLine.getCodePoint(1), 0);
+      expect(b.currentLine.getCodePoint(2), 0);
+      expect(b.currentLine.getCodePoint(3), 'b'.codeUnitAt(0));
+    });
+
+    test('deleteChars closes a gap at the cursor', () {
+      final b = _newBuffer(viewWidth: 5);
+      b.write('abcde');
+      b.setCursorX(1);
+      b.deleteChars(2);
+      expect(b.currentLine.getCodePoint(0), 'a'.codeUnitAt(0));
+      expect(b.currentLine.getCodePoint(1), 'd'.codeUnitAt(0));
+      expect(b.currentLine.getCodePoint(2), 'e'.codeUnitAt(0));
+    });
+
+    test('insertLines is a no-op outside the scroll region', () {
+      final b = _newBuffer(viewHeight: 5);
+      b.setVerticalMargins(2, 4);
+      b.setCursorY(0);
+      b.write('aaa');
+      b.insertLines(2);
+      // line 0 untouched (cursor was outside margins → no-op)
+      expect(b.lines[0].getCodePoint(0), 'a'.codeUnitAt(0));
+    });
+
+    test('insertLines inside the scroll region pushes lines down', () {
+      final b = _newBuffer(viewWidth: 3, viewHeight: 5);
+      b.setVerticalMargins(1, 3);
+      b.setCursor(0, 2);
+      b.write('aaa');
+      b.setCursor(0, 1);
+      b.insertLines(1);
+      // insertLines(1) pushes lines below the cursor down. So 'aaa'
+      // should now be at line 3.
+      expect(b.lines[3].getCodePoint(0), 'a'.codeUnitAt(0));
+      // Line 1 (cursor) is now empty.
+      expect(b.lines[1].getCodePoint(0), 0);
+    });
+
+    test('insertLines fills the entire region with empty lines when count >= lines below', () {
+      // When linesToInsert == linesBelow, linesToMove is 0 and the
+      // "fill empty" loop runs from 0 to linesToInsert.
+      final b = _newBuffer(viewWidth: 3, viewHeight: 5);
+      b.setVerticalMargins(1, 3);
+      b.setCursor(0, 1);
+      b.write('aaa');
+      b.setCursor(0, 2);
+      b.write('bbb');
+      b.setCursor(0, 1);
+      // 3 lines below (incl cursor) but request many more — clamps to 3
+      // and skips the "move" loop entirely.
+      b.insertLines(99);
+      for (var y = 1; y <= 3; y++) {
+        expect(b.lines[y].getCodePoint(0), 0);
+      }
+    });
+
+    test('deleteLines is a no-op outside the scroll region', () {
+      final b = _newBuffer(viewHeight: 5);
+      b.setVerticalMargins(2, 4);
+      b.setCursorY(0);
+      b.write('aaa');
+      b.deleteLines(2);
+      expect(b.lines[0].getCodePoint(0), 'a'.codeUnitAt(0));
+    });
+
+    test('deleteLines inside the scroll region pulls lines up', () {
+      // Use setCursor (not setCursorY) so cursorX is reset between
+      // writes — otherwise it stays saturated after the previous write.
+      final b = _newBuffer(viewWidth: 3, viewHeight: 6);
+      b.setVerticalMargins(1, 4);
+      b.setCursor(0, 2);
+      b.write('aaa');
+      b.setCursor(0, 3);
+      b.write('bbb');
+      b.setCursor(0, 1);
+      b.deleteLines(1);
+      // Inside the scroll region, deleteLines at line 1 shifts lines 2..4 up.
+      // Original content was at lines 2 ('aaa') and 3 ('bbb'); after deletion
+      // they should sit at 1 and 2.
+      expect(b.lines[1].getCodePoint(0), 'a'.codeUnitAt(0));
+      expect(b.lines[2].getCodePoint(0), 'b'.codeUnitAt(0));
+      expect(b.lines[4].getCodePoint(0), 0); // bottom of region cleared
+    });
+  });
+
+  group('Buffer — resize', () {
+    test('grow height pushes empty lines while there is room in the ring', () {
+      final b = _newBuffer(viewWidth: 5, viewHeight: 3, maxLines: 50);
+      b.resize(5, 3, 5, 5);
+      expect(b.height, 5);
+    });
+
+    test('grow height into existing scrollback bumps cursorY instead of pushing', () {
+      // Build up scrollback so lines.length > new viewHeight.
+      final b = _newBuffer(viewWidth: 3, viewHeight: 3, maxLines: 50);
+      // Force scrollback growth via index()-at-bottom on primary buffer.
+      b.setCursorY(2);
+      for (var i = 0; i < 5; i++) {
+        b.index();
+      }
+      expect(b.height, greaterThanOrEqualTo(8));
+      final cursorYBefore = b.cursorY;
+      // Resize to a height that's still smaller than lines.length: the
+      // grow loop should bump _cursorY rather than push new lines.
+      b.resize(3, 3, 3, 5);
+      expect(b.cursorY, greaterThan(cursorYBefore));
+    });
+
+    test('resize with reflow pads the result up to newHeight', () {
+      // Empty buffer: reflow returns near-zero lines; the pad-with-empty-
+      // lines branch fills up to newHeight.
+      final b = _newBuffer(viewWidth: 8, viewHeight: 3, reflowEnabled: true);
+      b.resize(8, 3, 4, 12);
+      expect(b.height, greaterThanOrEqualTo(12));
+    });
+
+    test('shrink height pops lines from the bottom when cursor is near the top', () {
+      final b = _newBuffer(viewWidth: 5, viewHeight: 5);
+      b.setCursorY(0);
+      b.resize(5, 5, 5, 3);
+      expect(b.height, 3);
+    });
+
+    test('shrink height drags cursor up when it sits below the new height', () {
+      final b = _newBuffer(viewWidth: 5, viewHeight: 5);
+      b.setCursorY(4);
+      b.resize(5, 5, 5, 3);
+      expect(b.cursorY, lessThanOrEqualTo(2));
+    });
+
+    test('resize without reflow forwards width to every line', () {
+      final b = _newBuffer(viewWidth: 5, viewHeight: 3, reflowEnabled: false);
+      b.resize(5, 3, 8, 3);
+      for (var y = 0; y < b.viewHeight; y++) {
+        expect(b.lines[y].length, 8);
+      }
+    });
+
+    test('resize with reflow on primary buffer pads the result up to newHeight', () {
+      final b = _newBuffer(viewWidth: 5, viewHeight: 3, reflowEnabled: true);
+      b.resize(5, 3, 8, 5);
+      expect(b.height, greaterThanOrEqualTo(5));
+    });
+  });
+
+  group('Buffer — scrollback / clear', () {
+    test('clearScrollback is a no-op when there is no scrollback', () {
+      final b = _newBuffer(viewHeight: 3);
+      b.clearScrollback();
+      expect(b.scrollBack, 0);
+    });
+
+    test('clearScrollback drops lines above the viewport', () {
+      final b = _newBuffer(viewHeight: 3);
+      // Force scrollback by indexing past the bottom.
+      b.setCursorY(2);
+      b.index();
+      b.index();
+      expect(b.scrollBack, 2);
+      b.clearScrollback();
+      expect(b.scrollBack, 0);
+    });
+
+    test('clear wipes everything and refills with viewHeight empty lines', () {
+      final b = _newBuffer(viewHeight: 3);
+      b.write('aaa');
+      b.clear();
+      expect(b.height, 3);
+      for (var y = 0; y < 3; y++) {
+        for (var x = 0; x < b.viewWidth; x++) {
+          expect(b.lines[y].getCodePoint(x), 0);
+        }
+      }
+    });
+  });
+
+  group('Buffer — anchors', () {
+    test('createAnchor at (x, y)', () {
+      final b = _newBuffer();
+      final a = b.createAnchor(2, 1);
+      expect(a.x, 2);
+      expect(b.lines[1].anchors, contains(a));
+    });
+
+    test('createAnchorFromOffset', () {
+      final b = _newBuffer();
+      final a = b.createAnchorFromOffset(const CellOffset(3, 2));
+      expect(a.x, 3);
+      expect(b.lines[2].anchors, contains(a));
+    });
+
+    test('createAnchorFromCursor', () {
+      final b = _newBuffer();
+      b.setCursor(4, 1);
+      final a = b.createAnchorFromCursor();
+      expect(a.x, 4);
+    });
+  });
+
+  group('Buffer — getWordBoundary', () {
+    test('returns null for an out-of-range y', () {
+      final b = _newBuffer(viewHeight: 3);
+      expect(b.getWordBoundary(const CellOffset(0, 99)), isNull);
+    });
+
+    test('returns null when the position is fully bounded by separators on both sides', () {
+      // The implementation walks left until a separator (or column 0) and
+      // right until a separator (or viewWidth). A null result requires the
+      // walks to land on the same column — i.e., separators flanking the
+      // position itself.
+      final b = _newBuffer(viewWidth: 5);
+      b.write('a   b'); // spaces at indexes 1, 2, 3
+      final r = b.getWordBoundary(const CellOffset(2, 0));
+      expect(r, isNull);
+    });
+
+    test('returns the surrounding word range, halting at separators', () {
+      final b = _newBuffer(viewWidth: 9);
+      b.write('foo bar');
+      final r = b.getWordBoundary(const CellOffset(5, 0))!;
+      expect(r.begin.x, 4);
+      expect(r.end.x, 7);
+    });
+
+    test('honours custom wordSeparators when provided', () {
+      final b = _newBuffer(viewWidth: 5, wordSeparators: <int>{','.codeUnitAt(0)});
+      b.write('a,bcd');
+      final r = b.getWordBoundary(const CellOffset(2, 0))!;
+      expect(r.begin.x, 2);
+      expect(r.end.x, 5);
+    });
+  });
+
+  group('Buffer — getText', () {
+    test('default range walks the whole buffer line-by-line with newlines', () {
+      final b = _newBuffer(viewWidth: 4, viewHeight: 3);
+      b.write('aa');
+      b.lineFeed();
+      b.write('bb');
+      final text = b.getText();
+      expect(text, contains('aa'));
+      expect(text, contains('bb'));
+      expect(text.split('\n').length, greaterThanOrEqualTo(2));
+    });
+
+    test('explicit range works on a normalized BufferRangeLine', () {
+      final b = _newBuffer(viewWidth: 4, viewHeight: 3);
+      b.write('aa');
+      b.lineFeed();
+      b.write('bb');
+      final r = BufferRangeLine(const CellOffset(0, 0), const CellOffset(2, 0));
+      expect(b.getText(r), 'aa');
+    });
+
+    test('skips out-of-range segments', () {
+      final b = _newBuffer(viewWidth: 4, viewHeight: 2);
+      b.write('aa');
+      // Range references a y past height; getText should skip it without
+      // throwing.
+      final r = BufferRangeLine(const CellOffset(0, 0), const CellOffset(2, 99));
+      final result = b.getText(r);
+      expect(result, contains('aa'));
+    });
+
+    test('wrapped lines do not get an inserted newline', () {
+      final b = _newBuffer(viewWidth: 3, viewHeight: 3);
+      b.write('abcdef'); // wraps line 1
+      final text = b.getText();
+      // The wrapped line shouldn't have a separating newline before it.
+      expect(text.replaceAll('\n', '').contains('abcdef'), isTrue);
+    });
+  });
+
+  group('Buffer — toString debug dump', () {
+    test('produces one line per buffer line with index and wrap marker', () {
+      final b = _newBuffer(viewWidth: 3, viewHeight: 3);
+      b.write('abcd'); // wraps; line 0 is 'abc', line 1 is 'd' (wrapped)
+      final out = b.toString();
+      expect(out, contains('|abc|'));
+      expect(out, contains('(⏎)')); // the wrapped marker
+    });
+  });
+}