Scale down MacOS scrolling from event controllers

[dtorop]

MacOS trackpad scroll events appear to be of much greater magnitude than those on other systems. To make them usable, we need to scale down the scroll deltas. For traditional GTK 3 code, dt_gui_get_scroll_unit_deltas() took care of this. Unfortunately, this code depends on the scroll-event signal. To make the code GTK 4 ready, we need to handle the scroll signal generated by a GtkEventControllerScroll.

For discrete scrolls, we can handle this transparently via the dt_gui_connect_scroll_discrete() setup function. This new function is similar to dt_gui_connect_scroll(). There is no need to call it with the GTK_EVENT_CONTROLLER_SCROLL_DISCRETE flag, as it adds this automatically. If darktable is compiled for MacOS, a proxy function will scale down the scroll events.

Note that this commit doesn't do anything to help scale down scroll events from a non-discrete GtkEventControllerScroll.

Also: In bauhaus _widget_scroll(), actually warn if called an a non-scroll event or no event rather than having a useless NOP conditional. Don't free the event in unlikely case we can't retrieve it.

Also: Update/remove a couple comments. In GTK4 it seems preferable to target events directly to widgets (as gtk_propagate_event and gtk_event_controller_handle_event are gone) so don't apologize in comments for using gtk_widget_event().

Fixes: #20698

See #20701 for discussion

[MStraeten]

no decrease of sensitivity on sliders and also scope (color harmony rotation) with this fix

[dtorop]

@MStraeten: Drat! So it doesn't fix anything?

Would you have time to add a bit of diagnostic code in and send the output when scrolling a slider? Something like:

modified   src/gui/gtk.c
@@ -4699,6 +4699,7 @@ static void _scroll_discrete_proxy(GtkEventControllerScroll* self,
                                     gdouble dy,
                                     _scroll_handler_proxy_t *h)
 {
+  dt_print_ext("proxy dx %f dy %f cur_dx %f cur_dy %f", dx, dy, h->cur_dx, h->cur_dy);
   // Even though the event controller has already accumulated smooth
   // scroll events to discrete dx/dy, MacOS smooth scrolling events
   // are too fast so scale them down via another accumulating stage.
@@ -4722,6 +4723,7 @@ static void _scroll_discrete_proxy(GtkEventControllerScroll* self,
       h->cur_dy -= steps;
       dy = steps;
     }
+    dt_print_ext("smooth dx %f dy %f cur_dx %f cur_dy %f", dx, dy, h->cur_dx, h->cur_dy);
   }
   if(dx != 0.0 || dy != 0.0)
     h->scroll_callback_real(self, dx, dy, h->scroll_data_real);

I also realize there might be a slightly nicer way to implement this (only accumulate scrolling at the darktable level, not at the GTK level), but not material unless this actually works.