1060971 - GTK3: Touchpad scrolling is not smooth

Status: RESOLVED WORKSFORME

(Core :: Widget: Gtk, defect)

Description

[Jan Alexander Steffens [:heftig]]

When scrolling using a touchpad, the frame rate is noticeably lower than when scrolling using a mousewheel or by dragging the scrollbar, making the scroll look jerky.

Firefox 33.0a2 built with GTK3, no OMTC or layer acceleration.
GNOME Shell 3.12, Haswell graphics, Arch Linux.

Comment 1

[Karl Tomlinson (:karlt)]

Bug used SCROLL_ASYNCHRONOUSELY for touchpad scrolling instead of the default which would have mapped to smooth scrolling.

Perhaps at least some touchpads don't send frequent enough events for this to be smooth.

Comment 2

[Jan Alexander Steffens [:heftig]]

Hmm, indeed the hardware (a Synaptics clickpad) seems to be the problem. Looking at the XI2 events, when using two-finger scrolling the deltas are always integer, while circular scrolling uses non-integer deltas and generates events at a higher rate. Circular scrolling also feels much smoother.

Comment 3

[Jan Alexander Steffens [:heftig]]

On the other hand, scrolling in other GTK3 apps like gtk3-demo or Epiphany isn't as bad as Firefox, so perhaps the lower event rate just exposes another problem.

For example, bug 958868 has a workaround for https://bugzilla.gnome.org/show_bug.cgi?id=726878 . Maybe that's not working quite as smooth as it should.

Comment 4

[Jan Alexander Steffens [:heftig]]

This has improved since replacing the xf86-input-synaptics driver with xf86-input-libinput. It's still not at 60fps, but I can no longer notice a difference between using the scroll bar and two-finger scrolling.