Comment 12 for bug 1762952

Just wanted to chime in on the upgrade path problem. I also upgraded from Ubuntu 17.10 and I also never set this shortcut anywhere. I have always used Super+Space and I have never used the Gnome Tweaks tool before. Using it to disable the shortcut fixed all the issues and the shortcuts using Shift - regardless of program - now work.

This seems to point to this being some kind of upgrade issue.