GdkScrollEvent reports raw scroll events from mouse wheels, 2-fingers touchpad motions, trackpoint scroll motions... Scroll events have different use cases. The most common one is to move content on screen. But they can also be used to switch weapon on a video-game, itterate values in a combo box or a number picker, zoom on a picture... Users ask for a way to configure the scroll speed. It only concerns the first most-common use case.
This MR adds gtk_event_controller_scroll_get_scroll_view_dx() and gtk_event_controller_scroll_get_scroll_view_dy() functions. After a scroll controller signal, these functions return the number of surface logical pixels to scroll. They need to be called if the event deserves the common "scroll view" use case. So the scrollable widget implementation will directly sum the returned value to its "pixels" scroll amount. The scroll velocity will depend on the scroll view dimensions. That's why they are asked in arguments.
We also have gtk_event_controller_scroll_get_scroll_view_decel_dx() and gtk_event_controller_scroll_get_scroll_view_decel_dy() to get the deceleration velocity of cumulated scroll view deltas after a decelerate signal.
This new API is here to inject the scroll speed settings in every GDK/GTK scrollable widgets implementations. It will also make the scroll behavior consistent accross all implementations.
I propose 3 scroll speed settings: mouse wheel, touchpad and trackpoints. On Wayland, it is mapped respectively to wl_axis_source.wheel, wl_axis_source.finger and wl_axis_source.continuous. The new API will look at the event GdkScrollUnit to determine the settings to use. We need a new unit to distinguish the wl_axis_source.continuous case. That's why I have added a new GDK_SCROLL_UNIT_CONTINUOUS enum. I have modified the GdkScrollUnit docs to precisely explain the meaning of each GdkScrollUnit enum.
At this time, the final public API is in place (although the nomenclature is still subject to change). The MR implements it in GtkScrolledWindow.
I still have to implement the new settings keys in The real settings are implemented.GtkSettings. For the moment, it's like the 3 scroll speed settings are hard coded to 1.0.