fix(sidebar): make file tree rows reliably clickable (#2368)

Native HTML5 drag starts after ~4px of pointer travel and suppresses the
click event for the gesture, so the draggable sidebar file tree rows
randomly ignored clicks on macOS trackpads/Magic Mouse: folders neither
expanded nor collapsed and files did not open.

Keep the rows draggable (dragging a file into the chat input inserts an
@mention) and recover the suppressed click on dragend: a drag that ended
within a small slop radius of its origin without dropping anywhere runs
the row's primary action. Also drop the misleading grab cursor so rows
read as clickable, matching FilesView.

Co-authored-by: Serhii Dziupin <makeittech@users.noreply.github.com>

packages/ui/src/components/layout/fileTreeDragClick.ts

@@ -0,0 +1,70 @@
+/**
+ * Click-reliability fallback for file tree rows that are both clickable and
+ * draggable (issue #2368).
+ *
+ * A native HTML5 drag starts after only a few pixels of pointer travel
+ * (4px in Chromium), and once `dragstart` fires the browser suppresses the
+ * `click` event for that gesture entirely. On macOS trackpads and Magic
+ * Mouse a plain click very often slips past that threshold, so rows that
+ * carry `draggable` (to drag file references into the chat input) randomly
+ * ignored clicks: folders neither expanded nor collapsed and files did not
+ * open.
+ *
+ * Arming `draggable` only after a pointer-move threshold is not a fix:
+ * Chromium decides drag eligibility on the first mouse move after mousedown
+ * and never re-evaluates, so a drag whose first movement stays below the
+ * threshold would never start (verified against headless Chromium).
+ *
+ * Instead the row stays draggable, and a drag that ends where it began —
+ * within a small slop radius and without dropping onto any target — is
+ * treated as the click it was meant to be. The two paths are mutually
+ * exclusive: when the browser suppresses `click` it fired `dragstart`, and
+ * when `click` fires no drag ever started, so the row action runs exactly
+ * once per gesture.
+ *
+ * Module-level state is safe here because the platform allows only one
+ * native drag at a time.
+ */
+
+/**
+ * Chromium starts a native drag at 4px of travel, so a suppressed click's
+ * dragstart→dragend distance is near zero. The slop only needs to absorb
+ * the remaining wobble between drag start and release; a deliberate drag
+ * released mid-flight travels far beyond it.
+ */
+const DRAG_CLICK_SLOP_PX = 8;
+
+type DragPointerEvent = {
+  clientX: number;
+  clientY: number;
+};
+
+let pendingDragOrigin: { x: number; y: number } | null = null;
+
+/** Record where a file row drag started. Call from the row's `dragstart`. */
+export const recordFileTreeDragStart = (event: DragPointerEvent): void => {
+  pendingDragOrigin = { x: event.clientX, y: event.clientY };
+};
+
+/**
+ * True when the drag that just ended was an accidental micro-drag that
+ * swallowed a click: it was never dropped onto a target and it ended within
+ * `DRAG_CLICK_SLOP_PX` of where it started. Consumes the recorded origin.
+ */
+export const shouldTreatFileTreeDragEndAsClick = (
+  event: DragPointerEvent & { dataTransfer: { dropEffect: string } | null },
+): boolean => {
+  const origin = pendingDragOrigin;
+  pendingDragOrigin = null;
+  if (!origin) return false;
+  if (event.dataTransfer && event.dataTransfer.dropEffect !== 'none') return false;
+  return (
+    Math.abs(event.clientX - origin.x) <= DRAG_CLICK_SLOP_PX
+    && Math.abs(event.clientY - origin.y) <= DRAG_CLICK_SLOP_PX
+  );
+};
+
+/** Reset module state. Intended for tests. */
+export const resetFileTreeDragClickState = (): void => {
+  pendingDragOrigin = null;
+};