Chrome 67.0.3369.0
Platform 10488.0.0
Termina 10473.0.0

1) Install gedit, open preferences window
2) Flip device into tablet mode
3) gedit goes fullscreen, but preferences window "jitters" by having its size constantly jump around by a few pixels

I've also seen this on the gnome-mahjongg scores window (top left menu, scores).

I guess this is some interaction with Chrome trying to resize the windows to be fullscreen. I think the intended behavior on the app side is fine, since these modal child windows don't need to be fullscreen. But the jumpy behavior while in tablet mode is a bit weird.