Weird resizing dance on Tor Browser startup

This is on Tails feature/jessie branch (GNOME Shell with the Classic set of extensions) with Tor Browser 4.5. Hardware is a Bay Trail tablet, with a 800 (horiz.) x 1280 (vert.) screen according to xrandr.

On startup, Tor Browser does a crazy resizing dance that I have a hard time describing. I'm pretty sure that the window is resized twice, or more. The warning about the dangers of maximizing the window is displayed, but it very quickly disappears (if I wasn't looking specifically for such things, I would have missed it).

Here are the logs that seemed relevant, that I could extract:

Window: (1,1) ?= (1212,800)
Window is normal
called init()
About to resize window: 300x371 inner: 300x300 in state 3 Available: 800x1212
Resized new window from: 800x1000 to 800x1000 in state 3
Mutation observer: Window dimensions are: 798x1000
Mutation observer: Window dimensions are (after resizing again): 800x1000

Once I manage to connect that system to the Internet, I'll report back about http://browserspy.dk/screen.php findings, as requested by Mike Perry.