1355375 - Scrollbar moves in opposite direction by 1px sometimes

Status: RESOLVED WORKSFORME

(Core :: Panning and Zooming, defect, P3)

Description

[684sigma]

I found a problem in Nightly 55. It doesn't happen in Beta 53.
Sometimes when scrolling a scrollable area up, scrollbar moves down, and vice versa. Here's how to reproduce it:

1. Open http://www.htmlgoodies.com/beyond/php/show-progress-report-for-long-running-php-scripts.html
2. Find the block of code containing text "//long_process.php". 
3. If it's not scrollable, add CSS rule "height: 105.4px !important;" to the div#highlighter_160787 that contains that block
4. Drag scrollbar up and down several times

Result: When mouse moves up, the block scrolls up by 1px, but scrollbar goes down.
When mouse moves down, the block scrolls down by 1px, but scrollbar goes up.

Expected: Scrollbar should follow mouse cursor

Comment 1

[684sigma]

Mozregression-gui generated this regression range:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=4cbc57e29e25e8abe783b16915bd74325a040b02&tochange=d7518eef6b8924b4ddad6cdeeae4eed1c6129b26
->
1324117 – Enable APZ scrollbar dragging on Nightly
https://bugzilla.mozilla.org/show_bug.cgi?id=1324117

Comment 2

[Kartikaya Gupta (email:[e-mail])]

This is nightly only, requires apz.drag.enabled. I can repro on Windows.

Comment 3

[Mike Taylor [:miketaylr]]

(marking as fix-optional for 55 since this is a nightly-only feature and blocks 1352312, which is the bug to ship by default)

Comment 4

[Mike Taylor [:miketaylr]]

(oops... blocks 1211610)

Comment 5

[Botond Ballo [:botond]]

684sigma, do you still see this problem?

When I try follow the STR today, I'm finding that adding the rule "height: 105.4px !important;" no longer makes the div scrollable. Changing that to "105" instead of "105.4" does, but the scrolling behaviour is then correct, i.e. it moves in the right direction.

Comment 6

[Kartikaya Gupta (email:[e-mail])]

I verified that this appears fixed. I can still repro the problem on the 2017-04-11 build but cannot repro it in the latest nightly.