Users reported laggy scrolling on Android after a recent Discourse update, affecting both mobile and desktop views, PWA, and browser. Don first identified the issue on a Huawei P20 Pro running Android 10 and Chrome 96, noting delayed scroll response after touch. david initially suspected a recent commit related to passive event listeners but found it didn’t resolve the issue — even after testing a theme that removed those listeners Don. Falco couldn’t reproduce the issue, but Osama confirmed it on a Pixel 4 XL with Android 12, especially noticeable during short, repeated scrolls. david then deployed a fix 4d3eb3f1ac that improved performance, which Don confirmed resolved the lag. david later realized the root cause was a separate commit 1166afa4e8 that refactored --header-offset logic to run continuously, slowing scroll. The fix optimized that logic, restoring smooth scrolling.
Hello,
With the latest update 1b5f26e0ab I used all day and the problem appear yesterday night. So I think the issue in one of the last few commits.
It seems to happens when I touch and start scrolling. The scrolling is not start immediately.
The issue seems to only on Android on both view (mobile and desktop) and the PWA and browser too.
It affect on the whole site where scrolling available.
My device:
Android 10
Chrome 96.0.4664.45
Thanks 
This is very interesting, thanks for reporting! As far as I’m aware, the only recent commit which affects scrolling would be:
But if anything, I would have expected that to make scrolling smoother, not worse! (I switched this particular feature to use ‘passive’ event listeners, rather than blocking event listeners)
Do you have the same problem on try.discourse.org?
As an experiment, I just made a theme here on Meta which removes those updated event listeners. https://meta.discourse.org/?preview_theme_id=260 (Named “Experiment - remove scroll listener” in the hamburger) Does this theme fix the lagging for you?
Hi David,
Thank you for the quick response. Unfortunately there is the same issue on try and the Experiment - remove scroll listener theme too. 
Thanks for checking! In a way I’m glad that didn’t fix it, because I have no idea why that change would cause worse performance! 
I’ll do another comb through recent commits and see if I can spot anything suspicious 
Would you mind letting us know the make/model of your device, so I can try to reproduce in a simulator?
On second thought, yes, it is positive 
Sure my device is a Huawei P20 Pro
Cant repro this on my phone on either Chrome or Firefox.
I can repro here on Meta using my Pixel 4 XL running Android 12. I didn’t notice it until I read this topic. The best way to see it is to scroll short distances (~2cm on the screen) repeatedly.
I made this improvement:
https://github.com/discourse/discourse/commit/4d3eb3f1acfc307e808c4f788c8683cc5c2da316
Which is now deployed to meta. Does that feel any better @Don?
That issue isn’t new though, so I’m not sure why it only started being noticeable in the last couple of days 
David now it is perfect Thank you!
I saw yesterday a commit about software update prompt Maybe that is related with this issue?
Glad to hear it!
Oh yes, you’re absolutely right! I totally missed that. This commit from yesterday refactored the --header-offset logic so that it runs all the time, rather than ‘only when docked’. That slowed down scroll performance. My fix made that logic more efficient, so that’s why it’s working now. 