Clicking on the tempo text input or the pencil button next to it causes it to retain the keyboard focus until Enter is pressed – even if the popup is dismissed.
Attempting to change the tempo by typing it in, followed by dismissing the popup by clicking outside its bounds, results in neither the tempo being changed nor the keyboard focus returning to the app. Once dismissed, it no longer seems to accept numerical entry, but it does listen for both Enter and ESC, when it finally updates the tempo (or not, depending) and returns the keyboard focus to the rest of the app.