After using the keyboard for a little bit, I added a few little fixes. I already mentioned moving some of the symbols keys to make them easier to access — the apostrophe is more common in English than the question mark or slash, so it makes sense to make it its own key. I also played a bit with key caps, just to make it a little bit nicer:

The second fix is the ability to use modifier keys with your mouse. Previously, when you held down a top/hold key, it wouldn't do anything until you released it or pressed any other key with it — then it would decide whether to send the modifier key code or the regular key code. But that makes it impossible to use this keyboard for things like ctrl-clicking or shift-dragging with your mouse. I initially tried to add a timeout to the hold function, but that didn't work very well — too slow for fast mouse clicking. Finally I made it so that it sends the modifier key as soon as you press the hold/tap key down, but if it turns out that you meant to tap it, it will release the modifier key before sending anything else. That way you get some extra spurious modifier key presses, but they normally do nothing, so it should be fine.