This is one of those glitches that is hard to pin down, but I’d like to report it anyway. On my mobile phone browser (Chrome, Android 6) it sometimes happens that the slide-up screen (coming up when you tap the progress bar) gets stuck, making the page unusable until I reload the page. I think it might happen mostly while composing, but I’m not sure. By “stuck” I mean that it won’t go away when I tap an empty area of the screen.
Another issue that I just had at the same time (though it may be unrelated) was that the message I had composed had disappeared after I navigated to an external page. I had been under the impression that discourse would still remember the autosave message, but in this case it was simply gone (already before I had to reload the page due to the slide-up thing and it did not come back after the reload either). The good news is that when I opened the same forum on my desktop computer, and did the same “reply as new topic” as I had originally done, my saved message magically reappeared! I think I did the same "reply as new topic"on my phone too, without the message reappearing, but I am not 100% sure that I did.
So while the saving of unfinished posts across devices and despite leaving the forum website is a killer feature, the mobile UX can still be improved.
And while I’m at it: I think the UX for new users could be improved by putting some kind of “close” icon or slide down icon onto that slide-up thing because it is easy to trigger it accidentally so that the user is confused and then gets even more confused when s/he doesn’t see a button to get rid of it again ( I would put it next to the notification settings button at the bottom).
Are you running any customizations or plugins on your Chrome Android? What version of Chrome Android? We only support the latest release not beta and certainly not canary.
The last couple of times it happened on https://community.letsencrypt.org/ Should have mentioned. Will watch and note if it happens anywhere else.
But even if this is a problem specific to that discourse instance, for whatever reason, I would say that it can happen again elsewhere and it would be good if discourse could be more robust on that point, given that it is quite a substantial UX failure. Maybe that button I suggested would already help, provided that it still works in that stuck-state.
Is some rare cases, scroll bugs can happen because of some buggy CSS.
Please, keep that in mind, and try to find a pattern. If it triggers mostly in some sites and never here on Meta (that’s almost vanilla Discourse) it’s a customization problem.
After we have a proper way to repro the bug we will fix it for sure.
Clicking above it doesn’t close it like it’s supposed to. The ways I’ve found to close it include:
Moving the timeline slider to a new post
Clicking the post title in the timeline view to jump to the first post (likely functioning same as if slider was dragged to post 1).
Does this help? I tested this in Chrome developer tools on Windows with an Android user agent. I can test on my phone too, but I use the back button when browsing on mobile, so I’m guessing this is how I’ve encountered it before.
The issue was we were sharing the instance of the topic timeline settings across renders. I’ve fixed it and now the timeline is collapsed when you enter a topic, even if you used the back button previously.
the mobile UX can still be improved.
or slide down icon 
onto that slide-up thing because it is easy to trigger it accidentally so that the user is confused and then gets even more confused when s/he doesn’t see a button to get rid of it again ( I would put it next to the notification settings button at the bottom).
! (I think… 
)