There is a gtk_event_controller_scroll_set_flags() call that's meant
to be called after construction (eg. due to scrolledwindow relayouts
hiding/showing scrollbars). The property shouldn't be construct-only
for consistence.
This is a GtkEventController implementation to handle mouse
scrolling. It handles both smooth and discrete events and
offers a way for callers to tell their preference too, so
smooth events shall be accumulated and coalesced on request.
On capable devices, it can also emit ::scroll-begin and
::scroll-end enclosing all ::scroll events for a scroll
operation.
It also has builtin kinetic scrolling capabilities, reporting
the initial velocity for both axes after ::scroll-end if
requested.
