OK I have a repro, this is the same bug reported to Apple in 2011
I have complete confidence it will be fixed sometime in the next 50 years when web browsers are obsolete and we are all jacked up to the Internet’s live feed 24x7. Probably in conjunction with the “free Internet” for people who opt-in for “dream based advertising”.
In the meantime feel free to raise an issue with Apple, I did in July, and it went into the Apple black hole.
We can work around this by yanking the entire page out of positioning when we show a modal and probably should, just like we do with the composer.
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This also impacts the new york times homepage as well as many other high profile sites.