Shift alone switches an input method in the merged keyboard setup
Pull request 14975 replays onto quattro a lone Shift that steps from an input engine such as Pinyin into Latin letters, and back. The same change had missed that branch. The merge is not in version 4.0.4.
The merged input configuration sets the alternate trigger to the left Shift key. Fcitx, the program that turns those keystrokes into characters, uses that key only while an engine is active, or to return to an engine that Shift just left. Shift plus a letter, and ordinary Latin typing, stay as they are.
A migration fills the key list only where an earlier migration left it empty. David Heinemeier Hansson wrote that the two commits are the ones from pull request 14940, replayed unchanged. That pull request merged into the branch caps-lock-with-shift-compose after the Caps Lock change had already merged, so the commits never reached quattro. He opened and merged pull request 14975.