I can reproduce in Firefox with a 1280x720 browser window, on meta, by picking Laptop in “Responsive Design Mode”, scrolling around, and noticing the scrollbar at the bottom appear and disappear as I do so. I notice that the scrollbar appears when I pass the bottom of the post stream, and does not appear while I’m in the middle of the post stream.
I can also reproduce in Chrome - from my testing - $(window).width() is 1000, $(window).height() is 540.
When scrolling past the last topic, the horizontal scrollbar appears and $(window).height() becomes 523. There is a “sweet spot” when the jumping occurs in which the horizontal scroll appears and disappears without user interaction in between the last topic and past the topic.
Yep, also mentioned in my report That’s the core of the problem. It looks like I can reproduce it up to about 1475px width. Once the composer “detaches” from the sides of the browser window, it goes away.