Wayland: implement transient scale factor windows as popups

as discussed in issue #794.
Also, fix for macOS the use of the FL_FOCUS event for these transient windows

src/Fl_Screen_Driver.cxx

@@ -390,14 +390,14 @@ struct WinAndTracker {
 
 static void del_transient_window(WinAndTracker *data) {
   delete (Fl_Image*)data->win->shape();
-  Fl::delete_widget(data->win);
+  delete data->win;
   if (data->tracker) {
     if (data->tracker->exists()) {
       data->tracker->widget()->take_focus();
     }
     delete data->tracker;
   }
-  delete data;
+  data->win = NULL;
 }
 
 void Fl_Screen_Driver::transient_scale_display(float f, int nscreen)
@@ -440,12 +440,18 @@ void Fl_Screen_Driver::transient_scale_display(float f, int nscreen)
   win->set_non_modal();
   Fl_Window_Driver::driver(win)->screen_num(nscreen);
   Fl_Window_Driver::driver(win)->force_position(1);
-  WinAndTracker *data = new WinAndTracker;
+  static WinAndTracker data = {NULL, NULL};
-  data->win = win;
+  if (data.win) {
+    Fl::remove_timeout((Fl_Timeout_Handler)del_transient_window);
+    delete data.win;
+  }
+  data.win = win;
   Fl_Widget *widget = Fl::focus();
-  data->tracker = (widget ? new Fl_Widget_Tracker(widget) : NULL);
+  if (!widget) widget = Fl::first_window();
+  data.tracker = (widget ? new Fl_Widget_Tracker(widget) : NULL);
   win->show();
-  Fl::add_timeout(1, (Fl_Timeout_Handler)del_transient_window, data); // delete after 1 sec
+  // delete transient win after 1 sec
+  Fl::add_timeout(1, (Fl_Timeout_Handler)del_transient_window, &data);
 }
 
 // respond to Ctrl-'+' and Ctrl-'-' and Ctrl-'0' (Ctrl-'=' is same as Ctrl-'+') by rescaling all windows

src/Fl_cocoa.mm

@@ -3144,7 +3144,8 @@ void Fl_Cocoa_Window_Driver::makeWindow()
   if (w->modal()) Fl::modal_ = w;
 
   w->set_visible();
-  if ( w->border() || (!w->modal() && !w->tooltip_window()) ) Fl::handle(FL_FOCUS, w);
+  if ( w->border() || (!w->modal() && !w->tooltip_window() &&
+                       w->user_data() != &Fl_Screen_Driver::transient_scale_display) ) Fl::handle(FL_FOCUS, w);
   [cw setDelegate:[FLWindowDelegate singleInstance]];
   if (show_iconic()) {
     show_iconic(0);

src/drivers/Wayland/Fl_Wayland_Window_Driver.cxx

@@ -1161,17 +1161,6 @@ static const struct xdg_popup_listener popup_listener = {
 bool Fl_Wayland_Window_Driver::in_flush_ = false;
 
 
-// Compute the parent window of the transient scale window
-static Fl_Window *calc_transient_parent(int &center_x, int &center_y) {
-  // Find top, the topmost window, but not a transient window itself
-  Fl_Window *top = Fl::first_window()->top_window();
-  while (top && top->user_data() == &Fl_Screen_Driver::transient_scale_display)
-   top = Fl::next_window(top);
-  center_x = top->w()/2; center_y = top->h()/2;
-  return top;
-}
-
-
 static const char *get_prog_name() {
   pid_t pid = getpid();
   char fname[100];
@@ -1255,6 +1244,8 @@ static const char *get_prog_name() {
  item, when there's one, is visible immediately after the tall popup is mapped on display.
  */
 
+static Fl_Window *transient_parent = NULL; // used for transient scale windows
+
 bool Fl_Wayland_Window_Driver::process_menu_or_tooltip(struct wld_window *new_window) {
   // a menu window or tooltip
   new_window->kind = Fl_Wayland_Window_Driver::POPUP;
@@ -1274,6 +1265,8 @@ bool Fl_Wayland_Window_Driver::process_menu_or_tooltip(struct wld_window *new_wi
       Fl_Window_Driver::menu_title(pWindow);
   }
   Fl_Widget *target = (pWindow->tooltip_window() ? Fl_Tooltip::current() : NULL);
+  if (pWindow->user_data() == &Fl_Screen_Driver::transient_scale_display &&
+      transient_parent) target = transient_parent;
   if (!target) target = Fl_Window_Driver::menu_parent();
   if (!target) target = Fl::belowmouse();
   if (!target) target = Fl::first_window();
@@ -1375,13 +1368,11 @@ void Fl_Wayland_Window_Driver::makeWindow()
 
   if (pWindow->user_data() == &Fl_Screen_Driver::transient_scale_display &&
       Fl::first_window()) {
-  // put transient scale win at center of top window by making it a child of top
+  // put transient scale win at center of top window by making it a tooltip of top
-    int center_x, center_y;
+    transient_parent = Fl::first_window();
-    Fl_Window *top = calc_transient_parent(center_x, center_y);
+    pWindow->set_tooltip_window();
-    if (top) {
+    pWindow->position((transient_parent->w() - pWindow->w())/2 ,
-      top->add(pWindow);
+                      (transient_parent->h() - pWindow->h())/2);
-      pWindow->position(center_x - pWindow->w()/2 ,  center_y - pWindow->h()/2);
-    }
   }
 
   if (pWindow->menu_window() || pWindow->tooltip_window()) { // a menu window or tooltip