It's been reported in several applications that scrolling feels jerky
since commit cc7b3985b3.
Investigation reported that the combination of passive 4-7 button grabs
on the toplevel and the presence of native subwindows might trigger
too often crossing events from the child window to the toplevel and
back as scroll "buttons" trigger the passive grab. Those crossing events
would reset the scroll valuators rendering scrolling from jerky on
touchpads (where there's intermediate smooth events between the emulated
button ones) to ineffective on regular mouse wheels (where the crossing
event would reset the valuators right before the single smooth scroll
event we get is delivered)
So, only reset scroll valuators when the pointer enters the toplevel
(we only care about this when the pointer is on the window after it's
been possibly scrolling somewhere else), and it doesn't come from an
inferior.
The situations where this happened varied though, the native subwindow
could be one created explicitly by the application, or created indirectly
through gdk_window_ensure_native(). The latter was mainly the case for
evolution (through gtk_selection_set_owner()) and any GtkScrolledWindow
under the oxygen-gtk3 theme (through gdk_window_set_composited())
https://bugzilla.gnome.org/show_bug.cgi?id=699574
4168c3cab9