Better fix for fake focus grab events

The fake ones have a special mode, so we can simply filter them before
they are passed on as FLTK events.

vncviewer/DesktopWindow.cxx

@@ -892,6 +892,17 @@ int DesktopWindow::fltkDispatch(int event, Fl_Window *win)
   if ((event == FL_MOVE) && (win == nullptr))
     return 0;
 
+#if !defined(WIN32) && !defined(__APPLE__)
+  // FLTK passes through the fake grab focus events that can cause us
+  // to end up in an infinite loop
+  // https://github.com/fltk/fltk/issues/295
+  if ((event == FL_FOCUS) || (event == FL_UNFOCUS)) {
+    const XFocusChangeEvent* xfocus = &fl_xevent->xfocus;
+    if ((xfocus->mode == NotifyGrab) || (xfocus->mode == NotifyUngrab))
+      return 0;
+  }
+#endif
+
   ret = Fl::handle_(event, win);
 
   // This is hackish and the result of the dodgy focus handling in FLTK.
@@ -1040,24 +1051,6 @@ void DesktopWindow::fullscreen_on()
     fullscreen();
 }
 
-#if !defined(WIN32) && !defined(__APPLE__)
-Bool eventIsFocusWithSerial(Display* /*display*/, XEvent *event,
-                            XPointer arg)
-{
-  unsigned long serial;
-
-  serial = *(unsigned long*)arg;
-
-  if (event->xany.serial != serial)
-    return False;
-
-  if ((event->type != FocusIn) && (event->type != FocusOut))
-    return False;
-
-  return True;
-}
-#endif
-
 bool DesktopWindow::hasFocus()
 {
   Fl_Widget* focus;
@@ -1105,11 +1098,6 @@ void DesktopWindow::grabKeyboard()
 #else
   int ret;
 
-  XEvent xev;
-  unsigned long serial;
-
-  serial = XNextRequest(fl_display);
-
   ret = XGrabKeyboard(fl_display, fl_xid(this), True,
                       GrabModeAsync, GrabModeAsync, CurrentTime);
   if (ret) {
@@ -1123,16 +1111,6 @@ void DesktopWindow::grabKeyboard()
     }
     return;
   }
-
-  // Xorg 1.20+ generates FocusIn/FocusOut even when there is no actual
-  // change of focus. This causes us to get stuck in an endless loop
-  // grabbing and ungrabbing the keyboard. Avoid this by filtering out
-  // any focus events generated by XGrabKeyboard().
-  XSync(fl_display, False);
-  while (XCheckIfEvent(fl_display, &xev, &eventIsFocusWithSerial,
-                       (XPointer)&serial) == True) {
-    vlog.debug("Ignored synthetic focus event cause by grab change");
-  }
 #endif
 
   keyboardGrabbed = true;
@@ -1159,19 +1137,7 @@ void DesktopWindow::ungrabKeyboard()
   if (Fl::grab())
     return;
 
-  XEvent xev;
-  unsigned long serial;
-
-  serial = XNextRequest(fl_display);
-
   XUngrabKeyboard(fl_display, CurrentTime);
-
-  // See grabKeyboard()
-  XSync(fl_display, False);
-  while (XCheckIfEvent(fl_display, &xev, &eventIsFocusWithSerial,
-                       (XPointer)&serial) == True) {
-    vlog.debug("Ignored synthetic focus event cause by grab change");
-  }
 #endif
 }