Startup window get letterboxed on MacOS in some screen resolutions (regression)

Startup window (but not the windows opened after that one) with some display / resolutions / scaling factors reports a fractional height for the browser container element, causing the content to be unnecessarily letterboxed vertically.

We introduced subpixel measurements to cope with rounding errors while still using margins to shrink the content, and this is probably the cause of the regression (even though it puzzles me that only the first window gets this weird height, which is the root cause).

However, now that we directly set the size of the browser element we can go back using integer sizes and therefore remove this regression.

/cc @thorin for additional QA on (unlikely) effectiveness regressions (I can check by myself on my MacBook whether this usability regression is fixed).