Same problem here on Chrome 74.0, MacOS 10.13.6, as well as Firefox 67.0. But it’s been like this for me for around a year, as long as I’ve been using the inbox, so on many previous versions of both browsers as well as the current ones. It happens without any extensions running on both browsers, and (for what it’s worth) on different internet connections, so connection speed shouldn’t be a concern.
Assuming this is in fact the same bug, it might help to add that this happens when at the oldest item in the inbox (but if there are only 1-2 notifications in the inbox, that can be above the fold.)
It happens in both Chrome and Firefox, but it manifests in slightly different ways: in Chrome the scroll bar disappears and reappears, which changes the width available for text. In Firefox, the scrollbar jumps up to the middle of it’s range, then back down to the bottom. It flickers back and forth every around twice per second for ~30 seconds, then finally settles. It makes the bottom of the inbox completely unreadable when first loaded.
I’ve emailed about this before but never got a response, unfortunately. And a coworker has the same trouble, but emailed & was told the best fix was to full-screen tasks in order to completely hide the inbox. It drives me crazy and slows my work down daily, so I would really love a fix for this!