Bug 1476195 - Allow invoking drag sessions outside of mouseDragged. r=mstange,spohl

The parent process usually starts a native drag session during the processing
of a Gecko mouse move event while the mouse is down. Usually, these Gecko mouse
move events are processed synchronously during -[ChildView mouseDragged:]. But
in some cases, the Gecko mouse move event can be a synthetic mouse move event
that was generated in response to a reflow. Those get processed during refresh
driver ticks, which run at a time that's completely unrelated to when
mouseDragged is invoked.
So the widget should just assume that drags can be started at any time between
mouseDown and mouseUp.

Differential Revision: https://phabricator.services.mozilla.com/D36151

--HG--
extra : moz-landing-system : lando

widget/cocoa/nsChildView.mm

@@ -3078,9 +3078,10 @@ NSPasteboard* globalDragPboard = nil;
 
 // gLastDragView and gLastDragMouseDownEvent are used to communicate information
 // to the drag service during drag invocation (starting a drag in from the view).
-// gLastDragView is only non-null while mouseDragged is on the call stack.
-NSView* gLastDragView = nil;
-NSEvent* gLastDragMouseDownEvent = nil;
+// gLastDragView is only non-null while a mouse button is pressed, so between
+// mouseDown and mouseUp.
+NSView* gLastDragView = nil;             // [weak]
+NSEvent* gLastDragMouseDownEvent = nil;  // [strong]
 
 + (void)initialize {
   static BOOL initialized = NO;
@@ -3321,6 +3322,10 @@ NSEvent* gLastDragMouseDownEvent = nil;
   [mPixelHostingView removeFromSuperview];
   [mPixelHostingView release];
 
+  if (gLastDragView == self) {
+    gLastDragView = nil;
+  }
+
   [super dealloc];
 
   NS_OBJC_END_TRY_ABORT_BLOCK;
@@ -4235,6 +4240,7 @@ NSEvent* gLastDragMouseDownEvent = nil;
 
   [gLastDragMouseDownEvent release];
   gLastDragMouseDownEvent = [theEvent retain];
+  gLastDragView = self;
 
   // We need isClickThrough because at this point the window we're in might
   // already have become main, so the check for isMainWindow in
@@ -4287,6 +4293,8 @@ NSEvent* gLastDragMouseDownEvent = nil;
 - (void)mouseUp:(NSEvent*)theEvent {
   NS_OBJC_BEGIN_TRY_ABORT_BLOCK;
 
+  gLastDragView = nil;
+
   if (!mGeckoChild || mBlockedLastMouseDown) return;
   if (mTextInputHandler->OnHandleEvent(theEvent)) {
     return;
@@ -4369,8 +4377,6 @@ NSEvent* gLastDragMouseDownEvent = nil;
     return;
   }
 
-  gLastDragView = self;
-
   WidgetMouseEvent geckoEvent(true, eMouseMove, mGeckoChild, WidgetMouseEvent::eReal);
   [self convertCocoaMouseEvent:theEvent toGeckoEvent:&geckoEvent];
 
@@ -4378,7 +4384,6 @@ NSEvent* gLastDragMouseDownEvent = nil;
 
   // Note, sending the above event might have destroyed our widget since we didn't retain.
   // Fine so long as we don't access any local variables from here on.
-  gLastDragView = nil;
 
   // XXX maybe call markedTextSelectionChanged:client: here?
 

widget/cocoa/nsDragService.mm

@@ -252,13 +252,10 @@ nsresult nsDragService::InvokeDragSessionImpl(nsIArray* aTransferableArray,
   NS_OBJC_BEGIN_TRY_ABORT_BLOCK_NSRESULT;
 
   if (!gLastDragView) {
-    // gLastDragView is only set during -[ChildView mouseDragged:].
-    // InvokeDragSessionImpl is only called while Gecko processes a mouse move
-    // event. So if we get here with gLastDragView being null, that means that
-    // the mouse button has already been released, and mouseMoved is on the
-    // stack instead of mouseDragged. In that case we need to abort the drag
-    // because the OS won't know where to drop whatever's being dragged, and we
-    // might end up with a stuck drag & drop session.
+    // gLastDragView is non-null between -[ChildView mouseDown:] and -[ChildView mouseUp:].
+    // If we get here with gLastDragView being null, that means that the mouse button has already
+    // been released. In that case we need to abort the drag because the OS won't know where to drop
+    // whatever's being dragged, and we might end up with a stuck drag & drop session.
     return NS_ERROR_FAILURE;
   }