Steps to recreate the bug…
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Open this window in Brave. Scaling is default, fitted to screen width, landscape: https://fr.wikipedia.org/wiki/Sylvain_Gouguenheim
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Switch tabs to Proton Mail and click a pdf link in an email, e.g. https://copeinc.org/docs/CalvertLawRev2018.pdf
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Now we are back in Brave. Switch to the earlier tab, and the scaling of the webpage is now enlarged, perhaps to 150%.
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Switch back to Proton Mail, and the scaling is also now wrong, perhaps 200% at a guess.