Correctly round new windows when bookmarks toolbar is set to "Only Show on New Tab"

This has happened at least twice to me, but I have no idea how to reproduce it.

On first launch, the viewport in Tor Browser 13.0 is 900px high and unletterboxed. After a period of several days, an additional 30px appears to get added to the height of the overall window, increasing the viewport to 930px and triggering letterboxing.

Note: this is not due to changes in the height of the browser chrome eating into the viewport (e.g. the bookmarks bar being on or off), as the height of the entire window has increased by 30px.

Tor Browser will then reliably reopen itself at the new dimensions even when quit and relaunched. Wiping the TorBrowser-Data file from macOS' Application Support folder seems to reset the browser back to its original dimensions, however.

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