AppSwitcher: Some app thumbnails always show right scrolling arrow
This does not seem to be dependent on the number of open windows, their size or their position in the AppSwitcher. What however seems to be affecting this is the length of the window title. Try running
gedit ~/test (kill all running instances before) and check the AppSwitcher. It should not have a scrolling indicator. Now kill gedit, run
gedit ~/some-really-really-long-filename, check the AppSwitcher again and you should see an arrow.