the repeat rate is too high, at least some of the time, causing two pushes when I mean one. Strangely, I didn't see this for the first 20 puzzles but now it's almost every time.

Can you swap undo/redo and prev/next level? Z is usually the key for undo, and Q/E are often used for prev/next. Add a setting for the default display mode between letters, highlighted letters and arrows. The text may be funny, but I prefer to always see the arrows I can use. Not all keyboards are querty, for some Z is up. Just an observation, I don't think there's any easy way to fix this, or make the game work in other languages than English....

although it's running in vsync mode at 60 fps, my laptop is overheating running this. Can't imagine and good reason for that. needs to go into idle between frames. perhaps you spend a lot of time polling the KB. which game engine is this?

I just find scanning the letters over and over and then looking over at my keyboard to be tiring. I realize the % sign changes them to arrows but then you lose the fun of them being letters. here's an alternate suggestion: how about a little red dot on the side of the letter that it moves toward? IE a D would have a small dot just to the right of the curve. Or a whole line along the side that it moves toward. another graphical suggestion: faint checkerboard for the background to help line up the letters ...

https://clips.twitch.tv/SpinelessTransparentPlumageCorgiDerp-j9qjKyIsDX3gTe2i