Ribbon Item Dropdown Problem

[steveh8204]

I've recently started having a problem selecting from drop down menus in the ribbon bar (changing Fill/Font colour etc).

When I click on the bottom corner to expand the menu it only half comes up and won't let me select anything (see pic).

https://drive.google.com/file/d/1lekq92fabvyKzHqDjWXzoBFnCpfPna_F/view?usp=sharing

I thought maybe it was a resource issue so closed everything else but it still persists. Not all the time but at the moment its every time I try. I've noticed its usually when I'm selecting multiple columns or rows.

Saving, closing and reopening doesn't work either.