Fix re-entrant exclusive-access crash in drag handle hit test (#771)

When sibling.hitTest() triggers a SwiftUI layout pass during the
drag handle's sibling walk, AppKit can call back into
windowDragHandleShouldCaptureHit before the outer invocation
finishes. This re-entry accesses SwiftUI view state that is already
held exclusively, causing a Swift runtime SIGABRT.

Add a module-level re-entrancy guard that bails out (returns false)
on nested calls to the sibling walk. Since hitTest is always called
on the main thread, a simple Bool flag is sufficient.

Crash was reproduced on macOS Sequoia 15.1.1 (24B91) in a UTM VM.
The crash stack: DraggableView.hitTest -> windowDragHandleShouldCaptureHit
-> sibling.hitTest -> SwiftUI body evaluation -> hitTest (re-entry)
-> exclusive-access violation -> SIGABRT.

Sources/WindowDragHandleView.swift

@@ -218,6 +218,12 @@ func windowDragHandleShouldTreatTopHitAsPassiveHost(_ view: NSView) -> Bool {
     return false
 }
 
+/// Re-entrancy guard for the sibling hit-test walk. When `sibling.hitTest()`
+/// triggers SwiftUI view-body evaluation, AppKit can call back into this
+/// function before the outer invocation finishes, causing a Swift
+/// exclusive-access violation (SIGABRT). Main-thread only, no lock needed.
+private var _windowDragHandleIsResolvingSiblingHits = false
+
 /// Returns whether the titlebar drag handle should capture a hit at `point`.
 /// We only claim the hit when no sibling view already handles it, so interactive
 /// controls layered in the titlebar (e.g. proxy folder icon) keep their gestures.
@@ -295,6 +301,20 @@ func windowDragHandleShouldCaptureHit(
         return true
     }
 
+    // Bail out if we're already inside a sibling hit-test walk. This happens
+    // when sibling.hitTest() re-enters SwiftUI layout, which calls hitTest on
+    // this drag handle again. Proceeding would trigger an exclusive-access
+    // violation in the Swift runtime.
+    guard !_windowDragHandleIsResolvingSiblingHits else {
+        #if DEBUG
+        dlog("titlebar.dragHandle.hitTest capture=false reason=reentrant point=\(windowDragHandleFormatPoint(point))")
+        #endif
+        return false
+    }
+
+    _windowDragHandleIsResolvingSiblingHits = true
+    defer { _windowDragHandleIsResolvingSiblingHits = false }
+
     let siblingSnapshot = Array(superview.subviews.reversed())
 
     #if DEBUG