ui/cocoa: Fix mouse grabbing in fullscreen mode for relative input de…

…vice

In fullscreen mode, the window property of cocoaView may not be the key
window, and the current implementation would not re-grab cursor by left click
in fullscreen mode after ungrabbed in fullscreen mode with hot-key ctrl-opt-g.

This patch used value of isFullscreen as a short-cirtuit condition for
relative input device grabbing.

Signed-off-by: Chen Zhang <tgfbeta@me.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 2D2F1191-E82F-4B54-A6E7-73FFB953DE93@me.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

ui/cocoa.m

@@ -902,7 +902,12 @@ - (bool) handleEventLocked:(NSEvent *)event
         case NSEventTypeLeftMouseUp:
             mouse_event = true;
             if (!isMouseGrabbed && [self screenContainsPoint:p]) {
-                if([[self window] isKeyWindow]) {
+                /*
+                 * In fullscreen mode, the window of cocoaView may not be the
+                 * key window, therefore the position relative to the virtual
+                 * screen alone will be sufficient.
+                 */
+                if(isFullscreen || [[self window] isKeyWindow]) {
                     [self grabMouse];
                 }
             }