Improve prioritization of event handlers added at open display time

2 files changed, 37 insertions, 7 deletions

diff --git a/src/Fl.cxx b/src/Fl.cxx
index 013d4dee1..c9a2972d2 100644
--- a/src/Fl.cxx
+++ b/src/Fl.cxx
@@ -827,6 +827,34 @@ void Fl::add_handler(Fl_Event_Handler ha) {
 }
 
 
+/** Returns the last function installed by a call to Fl::add_handler() */
+Fl_Event_Handler Fl::last_handler() {
+  return handlers ? handlers->handle : NULL;
+}
+
+
+/** Install a function to parse unrecognized events with less priority than another function.
+ Install function \p ha to handle unrecognized events
+ giving it the priority just lower than that of function \p before
+ which was previously installed.
+ \see Fl::add_handler(Fl_Event_Handler)
+ \see Fl::last_handler()
+ */
+void Fl::add_handler(Fl_Event_Handler ha, Fl_Event_Handler before) {
+  if (!before) return Fl::add_handler(ha);
+  handler_link *l = handlers;
+  while (l) {
+    if (l->handle == before) {
+      handler_link *p = l->next, *q = new handler_link;
+      q->handle = ha;
+      q->next = p;
+      l->next = q;
+      return;
+    }
+    l = l->next;
+  }
+}
+
 /**
  Removes a previously added event handler.
  \see Fl::handle(int, Fl_Window*)
diff --git a/src/Fl_Screen_Driver.cxx b/src/Fl_Screen_Driver.cxx
index d127b6b1e..112f376d3 100644
--- a/src/Fl_Screen_Driver.cxx
+++ b/src/Fl_Screen_Driver.cxx
@@ -571,21 +571,23 @@ void Fl_Screen_Driver::use_startup_scale_factor()
 void Fl_Screen_Driver::open_display()
 {
   static bool been_here = false;
-  // Add scale_handler first so it has least priority of all handlers
-  if (!been_here) Fl::add_handler(Fl_Screen_Driver::scale_handler);
-  open_display_platform();
   if (!been_here) {
     been_here = true;
-    bool keep_scale_handler = false;
+    open_display_platform();
+    // Memorize the most recently added handler. It may have been
+    // added by open_display_platform()
+    Fl_Event_Handler last_added = Fl::last_handler();
     if (rescalable()) {
       use_startup_scale_factor();
-      if (keyboard_screen_scaling && rescalable())
-        keep_scale_handler = true;
+      if (keyboard_screen_scaling && rescalable()) {
+        // Add scale_handler after memorized one in linked list
+        // so it has less priority
+        Fl::add_handler(Fl_Screen_Driver::scale_handler, last_added);
+      }
       int mx, my;
       int ns = Fl::screen_driver()->get_mouse(mx, my);
       Fl_Graphics_Driver::default_driver().scale(scale(ns));
     }
-    if (!keep_scale_handler) Fl::remove_handler(Fl_Screen_Driver::scale_handler);
   }
 }