feat: replace mouse/touch events with pointer events (#879)

feat: replace mouse/touch events with pointer events (#879)

- Use unified Pointer Events API for all drag interactions (mouse, touch, pen)
- Implement pointer capture for better drag UX (prevents cursor blinking)
- Add pointer ID filtering for multi-touch support
- Simplify useResizer hook by removing separate mouse/touch handlers
- Update DividerProps to use onPointerDown instead of onMouseDown/onTouchStart/onTouchEnd
- Update ResizeEvent source type from 'mouse' | 'touch' to 'pointer'
- Add PointerEvent polyfill for jsdom in test setup

Closes #878

Author: tomkp

src/components/Divider.tsx

@@ -42,9 +42,7 @@ export function Divider(props: DividerProps) {
     index,
     isDragging,
     disabled,
-    onMouseDown,
-    onTouchStart,
-    onTouchEnd,
+    onPointerDown,
     onKeyDown,
     className,
     style,
@@ -106,9 +104,7 @@ export function Divider(props: DividerProps) {
       tabIndex={disabled ? -1 : 0}
       className={combinedClassName}
       style={combinedStyle}
-      onMouseDown={disabled ? undefined : onMouseDown}
-      onTouchStart={disabled ? undefined : onTouchStart}
-      onTouchEnd={disabled ? undefined : onTouchEnd}
+      onPointerDown={disabled ? undefined : onPointerDown}
       onKeyDown={disabled ? undefined : onKeyDown}
       data-divider-index={index}
     >

src/components/SplitPane.tsx

@@ -273,13 +273,7 @@ export function SplitPane(props: SplitPaneProps) {
   );
 
   // Resizer hook
-  const {
-    isDragging,
-    currentSizes,
-    handleMouseDown,
-    handleTouchStart,
-    handleTouchEnd,
-  } = useResizer({
+  const { isDragging, currentSizes, handlePointerDown } = useResizer({
     direction,
     sizes: paneSizes,
     minSizes,
@@ -354,9 +348,7 @@ export function SplitPane(props: SplitPaneProps) {
             index={index}
             isDragging={isDragging}
             disabled={!resizable}
-            onMouseDown={handleMouseDown(index)}
-            onTouchStart={handleTouchStart(index)}
-            onTouchEnd={handleTouchEnd}
+            onPointerDown={handlePointerDown(index)}
             onKeyDown={handleKeyDown(index)}
             className={dividerClassName}
             style={dividerStyle}