Maybe you just stress the system too much and that causes hectic behaviour for display.
Not sure why you've chosen this particular exercise, but more than likely what you're witnessing is simply an artifact of the buffered IU updates being throttled while the system attempts to realize true state.
I'm mainly doing this to make sure that under stress the app still behaves correctly, but no, it's not hectic behaviour for the display or an artifact of the buffered UI. Moving or resizing the window after this doesn't redraw the text view correctly (the text view resizes during the live window resize, but it still leaves the same space blank). It's something happening inside the text view that makes it think that the text should occupy less space.
If this is some sort of durability spec on a given project, I'd log the results and see about negotiating something more representative of which the IDE may be better suited.
I don't know what IDE you're talking about. Do you mean my project, or Xcode?
I opened a bug report FB8745215.