I can't reproduce using the same FF build on mobile, or on desktop.

I can’t reproduce it either. Did you touch the screen when it flickered? Maybe your hand trembled, which was detected as many scrolls up (= show the button) and scrolls down (=hide the button). (By the way, it’s the new topic button, not the reply button. I was confused about what you meant before I watched the video.)

I does appear to be flickering very fast while the scroll isn't changing. I suspect some edge case rendering bug, maybe with non-integer scroll offsets. I tried changing the zoom level but still could not reproduce.

Here is a video with touch visibility enabled:
https://youtube.com/shorts/CzaVHHAsFpE?feature=share3

The videos are a bit misleading. During filming it flickers much more. It flickers so fast it appears semitransparent otherwise.

I can repeater this on Firefox nightly, without Auth. I can also repeat in private mode. This is on Samsung a52s.

In T330386#8641505, @Nux wrote:

Here is a video with touch visibility enabled:
https://youtube.com/shorts/CzaVHHAsFpE?feature=share3

It’s private, please make it public or unlisted (or something like that, I’m not sure about the YouTube terminology).

Done. I see recompressing on YT makes it even weirder. You have to imagine it flickers faster. Much faster. The flickering is always when I scroll up. Not touching the screen is not changing anything (still flickers).

I cannot reproduce on my old Moto G5 Plus.

I caught the moment when the new topic bar is hidden:

On desktop I can reproduce by changing bottom: -1px to bottom: 0.

Seems to be risky to use padding without box-sizing. Mixing em with px also might be problematic.

Thank you for filing this issue, @Nux.

Before prioritizing work on fixing this, we're going to see whether other people (specifically, people using browsers other than Firefox) people in experience a similar issue.

For context, ~0.7% of people use Firefox to edit on mobile devices:

Hardly a fair fight on Android with all those open-in-app modes and other stuff...

So I guess whatever. If anyone wants a fix then this seems to do the trick:
https://meta.wikimedia.org/w/index.php?title=User:Nux/global.css&oldid=24707697#L-15

I saw this today on a Wiktionary discussion page. So I set a breakpoint on attribute modification. It gets hit on line 102 of mobile.js:

			var observer = new IntersectionObserver(
				function ( entries ) {
					$newTopicWrapper.toggleClass( 'ext-discussiontools-init-new-topic-pinned', entries[ 0 ].intersectionRatio === 1 ); // <-- here
				},
				{ threshold: [ 1 ] }
			);

This line of code is both adding and removing the class. Adding the class triggers removal and vice versa.