Scrolling (using touchpad) in focused windows is smooth as expected. However, scrolling in unfocused windows is slow and laggy (with unexpected sudden stops and such) in some (not all) Gtk applications.

Some applications that show this behavior: vte/gnome-terminal with smooth scrolling patch (see below), gedit, yelp, gnome-font-viewer, gthumb's directory tree browser, evince's thumbnail bar, gucharmap's left bar, [gd]conf-editor's left bar.

The bug is only observable under certain window managers, including compiz, kde wm and icewm. It is not observable under certain others, such as gnome-shell, fvwm, windowmaker, fluxbox, blackbox.

Try vte with smooth scrolling, using the patch from bug 710426. Compile and run with these commands:
  ./configure --enable-debug
  make
  VTE_DEBUG=events ./src/vte2_90 -n 10000

You'll see that exactly under those window managers where it's buggy, the debug output contains tons of Leave and Enter messages right after each other. Based on this, my theory is:

- Some window managers send faulty Leave/Enter events when scrolling, even though the mouse clearly does not leave the window's area. I don't know if this behavior is legal or if all these WMs need to be fixed. I do wonder how come that at least 3 totally different WMs have this weird behavior.

- Probably GTK does something special in these events wrt scrolling, e.g. it dequeues a scroll event that was already queued to be develivered to the application, or something similar. I think it shouldn't do this, so that scrolling remains perfectly smooth even with spurious leave/enter events.

I'm not sure who the culprit is, please assign accordingly, or let me know if I need to file bugreports for the WMs. My first guess is Gtk since this is the single component that's common in all cases.

I'm using the touchpad of a Samsung NP300E5Z laptop, running Ubuntu Saucy 13.10.