Pressing shift only toggles input target sometimes

[tckmn]

In puzzles like Sudoku or Kakuro, you can input numbers into the four corners of empty cells. When using the keyboard, pressing shift should theoretically toggle which corner you're inputting into:

https://github.com/robx/pzprjs/blob/c26a5789b2a19010b5549c97b1a384b1acfde196/src/variety-common/KeyInput.js#L31

However, when I press shift, it only has a ~40% chance of working each time (??), and does nothing otherwise.

Firefox 71 on NixOS. (The issue is not present on Chromium 78.)

[robx]

As one further datapoint, Firefox 71 on macos seems to be fine.