Use keep_on_top on mac, as in GTK3/Quartz

`transient_for` forces the windows to be on the
same screen.

Also, removes the focus-follow-mouse behavior, as
it is adding too much complexity and its really
only needed to show the tooltip. Users are
responsible to focus the 2nd window if so they
want by clicking on it.

Author: masterpiga

src/views/darkroom.c

@@ -4102,10 +4102,6 @@ static gboolean _second_window_buttons_enter_notify_callback(GtkWidget *widget,
   gtk_widget_set_opacity(button_box, 1.0);
   gtk_overlay_set_overlay_pass_through(GTK_OVERLAY(gtk_widget_get_parent(button_box)),
                                        button_box, FALSE);
-  // GTK only shows tooltips for the focused window.  Focus the 2nd window when
-  // the pointer enters it so that tooltips work.  As a transient of the main
-  // window, the 2nd window always stays on top regardless of which has focus.
-  gtk_window_present_with_time(GTK_WINDOW(widget), event->time);
   return FALSE;
 }
 
@@ -4120,11 +4116,6 @@ static gboolean _second_window_buttons_leave_notify_callback(GtkWidget *widget,
     gtk_widget_set_opacity(button_box, 0.0);
     gtk_overlay_set_overlay_pass_through(GTK_OVERLAY(gtk_widget_get_parent(button_box)),
                                          button_box, TRUE);
-    // Return focus to the main window so keyboard shortcuts and other
-    // interactions work normally after leaving the 2nd window.
-    GtkWindow *main_window = gtk_window_get_transient_for(GTK_WINDOW(widget));
-    if(main_window)
-      gtk_window_present_with_time(main_window, event->time);
   }
   return FALSE;
 }
@@ -4319,8 +4310,14 @@ static void _darkroom_display_second_window(dt_develop_t *dev)
     gtk_window_set_icon_name(GTK_WINDOW(dev->second_wnd), "darktable");
     gtk_window_set_title(GTK_WINDOW(dev->second_wnd), _("darktable - darkroom preview"));
 
+#ifndef GDK_WINDOWING_QUARTZ
+    // On macOS, transient_for is implemented via [NSWindow addChildWindow:ordered:],
+    // which constrains the child to the parent's screen and prevents it from being
+    // moved to a different monitor.  Use keep_above instead (see below, after show_all,
+    // where the NSWindow is already realized).
     gtk_window_set_transient_for(GTK_WINDOW(dev->second_wnd),
                                  GTK_WINDOW(dt_ui_main_window(darktable.gui->ui)));
+#endif
 
     // Create the overlay for the window
     GtkWidget *overlay = gtk_overlay_new();
@@ -4369,6 +4366,13 @@ static void _darkroom_display_second_window(dt_develop_t *dev)
 
   // Show all widgets in the window
   gtk_widget_show_all(dev->second_wnd);
+
+#ifdef GDK_WINDOWING_QUARTZ
+  // keep_above must be set after the window is realized (i.e. after show_all),
+  // because the Quartz backend applies the NSWindow level change only to an
+  // already-existing NSWindow object.
+  gtk_window_set_keep_above(GTK_WINDOW(dev->second_wnd), TRUE);
+#endif
 }
 
 // clang-format off