When I added a sheet to display on top of TableFlips’ document, I wondered why the text field appear disabled, tabbing through elements didn’t work, and overall functionality was limited to accepting click events:

Everything appears disabled in this sheet

It turned out you have to make sure that you disable most of the NSWindow settings in Interface Builder except the title bar (NSTitledWindowMask). Only with a title bar (which is never visible in a sheet anyway) will the interaction work properly.

After ticking that checkbox, everybody’s happy:

Element focus works properly, including tabbing around and canceling with ESC

Sometimes, AppKit makes me crazy.