I have two essentially identical machines recently upgraded to xubuntu 18.04 386.
On the one machine the style, font and fontsize dropdowns are showing as plain text (no dropdown at all). Placing the cursor on the text and rotating the mouse wheel loops through the fonts/styles/sizes, but doesn't change selected text. All normal font functions are available via the menu (format>character) and seem to work correctly, so this is an issue with the toolbar dropdowns only. The other machine doesn't have this problem.
I've tried the normal things like resetting the user profile -doesn't help. There was an issue with this machine during upgrading to 18.04 in which some packages weren't correctly upgraded until a later post-install cleanup with aptitude (seemed to be OK, aptitude is now happy with the installation).
In LibreOffice 6 both machines needed the JRE disabled for Writer to start, but this is probably irrelevant to the dropdown problem.
Any ideas for fixing the dropdowns?