Darwin: Trap Deactivate messages and release modifiers to avoid "stuck shift lock" (etc) bugs

(cherry picked from commit 2b189a99330eb465fa0d17020fb1db1e38829151)

hw/darwin/darwinEvents.c

@@ -147,6 +147,16 @@ static void DarwinUpdateModifiers(
     }
 }
 
+/*
+ * DarwinReleaseModifiers
+ * This hacky function releases all modifier keys.  It should be called when X11.app
+ * is deactivated (kXDarwinDeactivate) to prevent modifiers from getting stuck if they
+ * are held down during a "context" switch -- otherwise, we would miss the KeyUp.
+ */
+static void DarwinReleaseModifiers(void) {
+	xEvent e;
+	DarwinUpdateModifiers(&e, KeyRelease, COMMAND_MASK(-1) | CONTROL_MASK(-1) | ALTERNATE_MASK(-1) | SHIFT_MASK(-1));
+}
 
 /*
  * DarwinSimulateMouseClick
@@ -347,6 +357,9 @@ void ProcessInputEvents(void) {
 	      ErrorF("Unexpected XDarwinScrollWheel event in DarwinProcessInputEvents\n");
 	      break;
 
+			case kXDarwinDeactivate:
+				DarwinReleaseModifiers();
+				// fall through
             default:
                 // Check for mode specific event
                 DarwinModeProcessEvent(&xe);