Created attachment 8013 Screenshot category list width is not resized when vertical scrollbar appears, causing category icons or text to be covered by scrollbar
As far as I can tell, this is caused by Gtk+3's aggravating decision to add overlay scrollbars. I don't set the width for the sidebar; it is handled by the GtkBox holding the buttons. Even when I explicitly disable overlay scrollbars in the code, Gtk+3 still doesn't make things wide enough. So far, the only way I have been able to find to prevent it overlapping the buttons is to always show the scrollbar, even when it is not needed. I'm still looking for an actual solution.
Graeme Gott referenced this bugreport in commit 4042c592fda301af56149bf03d701975516e3613 Fix sidebar scrollbar covering buttons. (bug #14730) https://git.xfce.org/panel-plugins/xfce4-whiskermenu-plugin/commit?id=4042c592fda301af56149bf03d701975516e3613