Paul E.C. Melis

Ah, you meant a GTK+ theme. Just checked: indeed, switching to a different theme in MATE changes the scrollbar behaviour. In the theme I'm using (TraditionalOK) a scrollbar gets shown regardless of the number of items in the completion list, which doesn't really work for a small number of items. Other themes apparently don't show a scrollbar until a larger number of items is in the list. So does that mean that the scrollbar behaviour is completely up to GTK+ and Scite has no influence over it? And...

Well, that was a good guess, it's definitely related to autocompletion. If I use autocompleteword.automatic=0 the issue goes away. When it's enabled I get a pretty useless scrollbar in the completion popup together with the error messages. But as far as I know I'm not using any particular theme. It's the vanilla package from the Arch AUR and I hadn't even heard of themes for Scite. So is this a bug in scite itself or a local issue?

Full output from --gtk-debug=geometry

Gtk-CRITICAL **: gtk_box_gadget_distribute: assertion 'size >= 0' failed in GtkScrollbar

Parameters passed to GLU_TESS_VERTEX(_DATA) callback are wrong

Sure, please send the patch.

Honestly, I find it strange that there is a perceptible difference between our Docker...

Okay, I did another test using the docker images you provided. Btw, my first foray...