I've been using Xfce since Gnome went crazy (3.0!) . . . and before that used KDE before it went crazy (plasma!) . . .
Somewhere along the way, I lost the ability to call the thesaurus in Libre/OpenOffice with Ctrl-F7. Didn't phase me too much, I just used the drop down menus to open it, looked smugly at the menu telling me "Ctrl-F7" would do the same thing, and went on my way . . .
Tonight I set to investigating the issue a bit, found numerous folks who can't get the thesaurus to work, or have it greyed out, etc. That wasn't me: the thesaurus was there and it worked -- a right click called up a mini-version, etc., -- only Ctrl-F7 would call it. The shortcut was defined in Tools->Customize->Keyboard, but nothing happened when the keys were pressed.
Turns out Xfce has Ctrl-F7 in use, and (logically) it takes precedence over the application.
What does Ctrl-F7 do? It helps users quickly, without delay, stroll over to their SEVENTH workspace!!! (Honestly, I almost never leave Workspace 1 . . . and when I do, it's usually an accident. I know workspaces have their place, but . . .)
Once I removed that little novelty . . . the LibreOffice keyboard command worked perfectly. The Xfce shortcut can be undone from the menu: Settings->Settings Manager->Window Manager->Keyboard tab. Scroll down the list. When you find Ctrl-F7, double-click on it, and choose "Clear."
Thought this might effect other programs short-cuts as well, so hopefully other Xfce desktop users can benefit from this.