panel splitters get keyboard parity and Semantics (T-111)

Drag handles for sidebar / context / editor split were pure
pointer-Listeners — no Tab focus, no arrow-key adjust, no Semantics.
Each now wraps in a FocusableActionDetector with arrow shortcuts (10
px fine / 50 px coarse for the column handles, 2% / 10% for the
editor split) and a slider Semantics node that announces the current
size. The CLI verb half is split out as T-119 and waits on T-99.

Co-Authored-By: Claude <noreply@anthropic.com>

lib/kernel/src/panels/drag_resize.dart

@@ -1,11 +1,16 @@
 import 'package:clide/kernel/src/panels/arrangement.dart';
 import 'package:clide/kernel/src/panels/slot_id.dart';
 import 'package:clide/kernel/src/theme/controller.dart';
+import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';
 
-/// A 4-px draggable splitter that adjusts the size of [slot] in the
-/// given [arrangement]. Slot hosts wrap this around their edges to make
-/// the three-column layout resizable.
+/// A 4-px splitter that adjusts the size of [slot] in [arrangement].
+/// Slot hosts wrap this around their edges to make the three-column
+/// layout resizable.
+///
+/// Drag with the mouse, or tab to it and use the arrow keys (Shift =
+/// coarse step). Exposes a `slider` Semantics node so screen readers
+/// announce the current width.
 class DragResizeHandle extends StatefulWidget {
   const DragResizeHandle({
     super.key,
@@ -21,6 +26,8 @@ class DragResizeHandle extends StatefulWidget {
   final double thickness;
 
   static const defaultThickness = 8.0;
+  static const double stepFine = 10.0;
+  static const double stepCoarse = 50.0;
 
   @override
   State<DragResizeHandle> createState() => _DragResizeHandleState();
@@ -28,32 +35,57 @@ class DragResizeHandle extends StatefulWidget {
 
 class _DragResizeHandleState extends State<DragResizeHandle> {
   bool _hovered = false;
+  bool _focused = false;
   double? _dragStartSize;
   Offset? _dragStartPointer;
 
   @override
   Widget build(BuildContext context) {
     final tokens = ClideTheme.of(context).surface;
-    final lineColor = _hovered ? tokens.panelActiveBorder : tokens.dividerColor;
+    final lineColor = (_hovered || _focused) ? tokens.panelActiveBorder : tokens.dividerColor;
 
-    return MouseRegion(
-      cursor: widget.axis == Axis.horizontal ? SystemMouseCursors.resizeColumn : SystemMouseCursors.resizeRow,
-      onEnter: (_) => setState(() => _hovered = true),
-      onExit: (_) => setState(() => _hovered = false),
-      child: Listener(
-        onPointerDown: _onDown,
-        onPointerMove: _onMove,
-        onPointerUp: _onUp,
-        child: Container(
-          width: widget.axis == Axis.horizontal ? widget.thickness : null,
-          height: widget.axis == Axis.vertical ? widget.thickness : null,
-          color: tokens.chromeBackground,
-          child: Align(
-            alignment: widget.slot == Slots.sidebar ? Alignment.centerRight : Alignment.centerLeft,
+    final size = widget.arrangement.sizeOf(widget.slot);
+
+    return Semantics(
+      container: true,
+      slider: true,
+      label: _semanticLabel(),
+      value: size == null ? null : '${size.round()} pixels',
+      increasedValue: size == null ? null : '${(size + DragResizeHandle.stepFine).round()} pixels',
+      decreasedValue: size == null ? null : '${(size - DragResizeHandle.stepFine).round()} pixels',
+      onIncrease: () => _bump(DragResizeHandle.stepFine),
+      onDecrease: () => _bump(-DragResizeHandle.stepFine),
+      child: FocusableActionDetector(
+        onShowFocusHighlight: (v) => setState(() => _focused = v),
+        shortcuts: _shortcuts(),
+        actions: <Type, Action<Intent>>{
+          _BumpIntent: CallbackAction<_BumpIntent>(
+            onInvoke: (intent) {
+              _bump(intent.delta);
+              return null;
+            },
+          ),
+        },
+        child: MouseRegion(
+          cursor: widget.axis == Axis.horizontal ? SystemMouseCursors.resizeColumn : SystemMouseCursors.resizeRow,
+          onEnter: (_) => setState(() => _hovered = true),
+          onExit: (_) => setState(() => _hovered = false),
+          child: Listener(
+            onPointerDown: _onDown,
+            onPointerMove: _onMove,
+            onPointerUp: _onUp,
             child: Container(
-              width: widget.axis == Axis.horizontal ? 1 : null,
-              height: widget.axis == Axis.vertical ? 1 : null,
-              color: lineColor,
+              width: widget.axis == Axis.horizontal ? widget.thickness : null,
+              height: widget.axis == Axis.vertical ? widget.thickness : null,
+              color: tokens.chromeBackground,
+              child: Align(
+                alignment: widget.slot == Slots.sidebar ? Alignment.centerRight : Alignment.centerLeft,
+                child: Container(
+                  width: widget.axis == Axis.horizontal ? (_focused ? 2 : 1) : null,
+                  height: widget.axis == Axis.vertical ? (_focused ? 2 : 1) : null,
+                  color: lineColor,
+                ),
+              ),
             ),
           ),
         ),
@@ -61,6 +93,43 @@ class _DragResizeHandleState extends State<DragResizeHandle> {
     );
   }
 
+  String _semanticLabel() {
+    final axis = widget.axis == Axis.horizontal ? 'width' : 'height';
+    if (widget.slot == Slots.sidebar) return 'Sidebar $axis';
+    if (widget.slot == Slots.contextPanel) return 'Context panel $axis';
+    return '${widget.slot.value} $axis';
+  }
+
+  Map<ShortcutActivator, Intent> _shortcuts() {
+    final horiz = widget.axis == Axis.horizontal;
+    final fine = DragResizeHandle.stepFine;
+    final coarse = DragResizeHandle.stepCoarse;
+    return <ShortcutActivator, Intent>{
+      if (horiz) ...{
+        const SingleActivator(LogicalKeyboardKey.arrowLeft): _BumpIntent(-fine),
+        const SingleActivator(LogicalKeyboardKey.arrowRight): _BumpIntent(fine),
+        const SingleActivator(LogicalKeyboardKey.arrowLeft, shift: true): _BumpIntent(-coarse),
+        const SingleActivator(LogicalKeyboardKey.arrowRight, shift: true): _BumpIntent(coarse),
+      } else ...{
+        const SingleActivator(LogicalKeyboardKey.arrowUp): _BumpIntent(-fine),
+        const SingleActivator(LogicalKeyboardKey.arrowDown): _BumpIntent(fine),
+        const SingleActivator(LogicalKeyboardKey.arrowUp, shift: true): _BumpIntent(-coarse),
+        const SingleActivator(LogicalKeyboardKey.arrowDown, shift: true): _BumpIntent(coarse),
+      },
+    };
+  }
+
+  /// Apply a raw delta in the natural axis direction. The drag handler
+  /// inverts the sign for context-panel because its handle sits on the
+  /// left edge of the right-anchored slot; arrow keys follow the same
+  /// convention so right-arrow always moves the boundary rightward.
+  void _bump(double rawDelta) {
+    final current = widget.arrangement.sizeOf(widget.slot);
+    if (current == null) return;
+    final delta = widget.slot == Slots.contextPanel ? -rawDelta : rawDelta;
+    widget.arrangement.setSize(widget.slot, current + delta);
+  }
+
   void _onDown(PointerDownEvent e) {
     _dragStartSize = widget.arrangement.sizeOf(widget.slot);
     _dragStartPointer = e.position;
@@ -80,3 +149,8 @@ class _DragResizeHandleState extends State<DragResizeHandle> {
     _dragStartPointer = null;
   }
 }
+
+class _BumpIntent extends Intent {
+  const _BumpIntent(this.delta);
+  final double delta;
+}