In Writer, I map Ctrl+Shift+u to the Edit/Update All function. Instead it prints an underlined, lower-case u. I'm in LO 5.1.2 on .deb linux.
Well, choose another mapping, or disable any keyboard IME you might have enabled, such as iBus (in Ubuntu, this can be done through the Language Support application). Otherwise, you’ll keep getting conflicts with GNOME’s Unicode key functionality [1] [2]. [1]: https://en.wikipedia.org/wiki/Unicode_input#In_X11_.28Linux_and_other_Unix_variants.29 [2]: https://developer.gnome.org/gtk3/unstable/GtkIMContext.html
Thank you, I was unaware of that. Case closed.