We don't apply it to System Principaled pages; but we do apply it to about: pages that are not system principaled; as well as moz-extension:// pages.

It's safe to not apply it to about: pages; and it seems like a reasonable choice to not apply it to Web Extension pages also.

I'd like to start uplifting our downstream letterboxing patches.
Can I reuse this Bug for other nice to have exceptions (pdf.js and view-source), or shall I create new ones?

It's fine to reuse this one unless you think the changes are significant enough/separate enough that it makes sense to make new ones.

Add the following exemptions to letterboxing:

• browser extensions
• view-source URLs
• pdf.js

Backed out for causing mochitests failures in browser_dynamical_window_rounding.js.

• Backout link
• Push with failures
• Failure Log
• Failure line: TEST-UNEXPECTED-FAIL | browser/components/resistfingerprinting/test/browser/browser_dynamical_window_rounding.js | Test timed out -

It took me a few attempts to reproduce, but then I got the error.
I sent an updated patch which hopefully will fix the problem (I tried the test several times and it always worked on my machine).
While doing so I found another possible problem with focus and fullscreen, fixed those as well.

Fatih, could you please re-land the corrected version for me?

Sure, I just submitted https://treeherder.mozilla.org/jobs?repo=try&revision=2c5c305c5050f24ee959cd2a5a32caff78e95ae7. I'll land it after it is done.

removing ni in the meantime so the bugbot is happy with me. feel free to re-ni.

https://hg.mozilla.org/mozilla-central/rev/0f2f5106a9c9

Adding tor 32411 due to this patch also excepting view-source: (and therefore d577f12a being uplifted here).