arni2033 Reporter
	Description • 9 years ago

>>>   My Info:   Win7_64, Nightly 46, 32bit, ID 20151223030323
STR:
1. Make sure that mouse option "When I rotate the mouse wheel" in your OS is set to  "scroll by 1 page"
2. Open attached "testcase 1" with slow animation
3. Press End to scroll the page to the bottom
4. Scroll mouse wheel up once
You must perform Steps 2-4 within 90 seconds.

Result:       
 The page twitches a bit, but Step 4 doesn't seem to do anything
 [request a screencast if necessary]

Expectations: 
 The page should be scrolled up

This was regressed between 2015-12-02 and 2015-12-03 by bug 1228028
> pushlog_url:   https://hg.mozilla.org/integration/mozilla-inbound/pushloghtml?fromchange=0dbac1acaca7ae2a767604fdbcf5105853518c88&tochange=f7ee42669b87f05416eb785ab9db27b5c66de1a3

	Alice0775 White
	Comment 1 • 9 years ago

I can reproduce the scroll problem with default mouse wheel amount(3 lines/a single tick).

Regression window:
https://hg.mozilla.org/integration/mozilla-inbound/pushloghtml?fromchange=738d8ee106fe&tochange=af67b5f73381

Regressed by af67b5f73381	Kartikaya Gupta — Bug 1157745 - Enable APZ on Windows desktop nightly builds. r=dvander

	Kartikaya Gupta (email:kats@mozilla.staktrace.com)
	Comment 2 • 9 years ago

The problem here is a variant of the problem in bug 1228407. What's happening is that the CSS animation is actually shortening the page very slowly. When you're scrolled to the very bottom, that means the scroll offset needs to move upwards to compensate for the page getting shorter. The main thread takes care of this by sending a flood of scroll position updates to the APZ, which keeps the scroll generation incrementing continuously and prevents APZ from doing any updates to the scroll offset. When you're not at the bottom of the page, the main thread doesn't need to keep updating the scroll offset and so this problem doesn't happen.

Need to think of a good fix to this that's more robust than what we did in bug 1228407.

	Kartikaya Gupta (email:kats@mozilla.staktrace.com)
	Comment 3 • 9 years ago

(And yes, this isn't specific to page scrolling, I was able to repro with line scrolling. Also it's a bit of an edge case so I won't block APZ on it).

	Kartikaya Gupta (email:kats@mozilla.staktrace.com)
	Comment 4 • 8 years ago

Marking as wontfix for 46; even if we fix it in Nightly I doubt we would uplift the fix to Beta.

	Kartikaya Gupta (email:kats@mozilla.staktrace.com)
	Comment 5 • 8 years ago

Not worth uplifting when we do fix it.

	Kartikaya Gupta (email:kats@mozilla.staktrace.com)
	Comment 6 • 8 years ago

Bumping again, low priority.