The upgrade screen. I couldn't figure out what caused it. There was nothing it the console logs.
It seemed to only happened after moving around the tree (wasd) but not every time.
Pressing escape/clicking the menu/back button did work where I could go back to the play button which would show me the last dialog I saw then bring me back to the upgrade screen. Space would work at this point (continue dialog and to play the game).
Esc also closes fullscreen, which was annoying... Neither clicking back in, clicking upgrades, nor entering full screen again would fix it.
That's all I can remember (Firefox on Linux)