fix(ui): treat every second click of a burst as a double-click

WebKit fires `dblclick` only for the second click of a burst. A quick
double-click right after another arrives as `click` with `detail` 3 and 4 and
no `dblclick`, so the second double-click on a folder in the Project view (and
on any tree row, tab or list item handled by `onDoubleClick`) did nothing.

A window-level shim now dispatches a `dblclick` on the click's target for every
later even click of a burst (detail 4, 6, ...), after the page's own click
handlers. Existing `onDoubleClick` handlers and their `stopPropagation` apply
unchanged; an engine that fires its own `dblclick` for that click gets it
dropped as a duplicate.

The Project view test now delivers bursts the way WebKit does (`dblclick` only
at detail 2) and checks both expand modes over 2–6 click bursts.

Refs #724

src/platform/doubleClick.ts

@@ -0,0 +1,55 @@
+/**
+ * WebKit (the macOS webview) fires `dblclick` only for the second click of a burst: a
+ * quick double-click right after another arrives as `click` with `detail` 3 and 4 and no
+ * `dblclick`, so the second double-click on a folder, tab or row did nothing. This turns
+ * every later even click of a burst into a `dblclick` on the same target, so React
+ * `onDoubleClick` handlers (and their `stopPropagation`) see each double-click. An engine
+ * that does fire its own `dblclick` for that click gets it suppressed as a duplicate.
+ */
+const synthetic = new WeakSet<Event>();
+
+export function installRepeatedDoubleClick(win: Window = window): () => void {
+  let last: { target: EventTarget; detail: number } | null = null;
+
+  // Bubble phase on the window: after the page's own click handlers, like the engine's dblclick.
+  const onClick = (e: MouseEvent) => {
+    last = null;
+    if (e.button !== 0 || e.detail < 4 || e.detail % 2 !== 0 || !e.target) return;
+    const dbl = new MouseEvent("dblclick", {
+      bubbles: true,
+      cancelable: true,
+      composed: true,
+      view: e.view,
+      detail: e.detail,
+      screenX: e.screenX,
+      screenY: e.screenY,
+      clientX: e.clientX,
+      clientY: e.clientY,
+      button: e.button,
+      buttons: e.buttons,
+      ctrlKey: e.ctrlKey,
+      shiftKey: e.shiftKey,
+      altKey: e.altKey,
+      metaKey: e.metaKey,
+    });
+    synthetic.add(dbl);
+    last = { target: e.target, detail: e.detail };
+    e.target.dispatchEvent(dbl);
+  };
+
+  const onDoubleClick = (e: MouseEvent) => {
+    if (synthetic.has(e) || !last) return;
+    const duplicate = e.detail === last.detail && e.target === last.target;
+    last = null;
+    if (!duplicate) return;
+    e.stopImmediatePropagation();
+    e.preventDefault();
+  };
+
+  win.addEventListener("click", onClick);
+  win.addEventListener("dblclick", onDoubleClick, true);
+  return () => {
+    win.removeEventListener("click", onClick);
+    win.removeEventListener("dblclick", onDoubleClick, true);
+  };
+}