Writing prevents tab switching

[sorawee]

Steps to reproduce:

1. In an empty buffer, evaluate (make-string 100000 #\A)
2. Cmd+T to create a new buffer in a new tab and switch to it.
3. Attempt to switch back to the previous buffer.

Instead of being able to switch back to the previous buffer right away, DrRacket blocks until printing is completely done first.

[rfindler]

I guess that probably what's happening is that there are just a lot of GUI events stacked up and the switch-tab click that you've done has a lower priority (or is behind in the queue at the same priority? but probably not) than the GUI work that's going on to do all the IO.