Fixed window size with sidebar causes window judder at startup

Steps to reproduce:

1. Toggle on "General > Browser Layout > Show sidebar".
2. Make sure the Letterboxing preference "Reuse last window size when opening a new window" is off.
3. Restart the browser.

At startup the window width will be wider for a few frames, and then will shrink down. At the new size letterboxing is visible on web pages.

In contrast, any new windows opened in the same session will stay at the wider width and will not have letterboxing visible. So it only seems to be an issue with the initial window.

/cc @donuts @ma1