Okay, this is a new one for me, but I've been thinking on it a little.
On my phone, the very top black bar moves up and down/hides when scrolling at the top of the page, but it's anchored at the top, so it doesn't end up in the middle of the page like that. In trying to rule things out, have you tried any or all of the following things?
-First, reboot your phone. Rebooting commonly fixes a large number of weird things, and most people rarely bother to do it until the phone locks up or something. My mantra... When in doubt, reboot. Applies to virtually all things computer related, across all operating systems.
-Completely close out the browser and re-open to see if anything changes.
-Close out the browser completely, go to app settings on your phone for the browser, and clear the cache, then restart the browser and see if that solves it.
-Try using a different browser app. Possible what's happening is a browser specific coding thing, and the glitch may be the result of a recent update. I use different browsers on different devices because some of them don't format certain websites properly. If the problem doesn't happen on a different browser, there's your answer.
I know there's a current discussion about relative safety of different browsers, but the point is largely moot. They ALL use your browsing habits and history. Some are just more transparent about doing so than others. We sign away rights simply by using the devices we've become so dependent on. It's in the user agreements. In reality, the only way to completely protect your information is to not use the devices at all, so we weigh the convenience against the risk in our decision to use them.
So, on my android phone, Chrome seems to work best for me for forum and mod related stuff. I manage 90+% of my general forum activities and modding on my phone. I can't really speak about apple because I don't have an iPhone, but on my son's iPad, I've found it difficult to do much of what I need for moderating. Regular posting is okay. On PC, much though I dislike Microsoft, Edge browser works best for me in terms of functionality and page formatting on the forum. I use Firefox for almost everything else. Again, can't speak on Macs as I don't have one.
Let me know if any of this helps. I kind of enjoy problem solving, so now I'm curious.