Press tab editing a sign; can't move focus away from the Done button

When in the sign editing interface, if I press tab while I'm editing the sign, the "Done" button gets focus, as expected. However, there is no way to move focus away from the button. Pressing tab (or shift+tab) again doesn't work, nor does clicking the mouse elsewhere. I can use the mouse to put the cursor back on the sign, and continue editing it, but as soon as I press space, the UI closes since the button still has focus.