1349236 - Fast scrolling up and down by dragging the scrollbar doesn't work properly

Status: RESOLVED INVALID

(Core :: Panning and Zooming, defect)

Description

[ovidiu boca[:Ovidiu]]

Tested on Mac OS X 10.10, MacBook Pro 10.12, Windows 10x64 using Nightly 55.0a1(2017-03-21).

STR: 

1. Open Firefox and go to:
https://staktrace.com/resources/extras/spout/scroll-behavior.html
2. Reduce the browser window size (you need to do that to obtain a longer scrollbar)
3. Drag the scrollbar and move it fast up and down. 
 
Expected result: The text is properly displayed, without any delay.

Actual result: The text it has some delay in displaying the context.

Comment 1

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

Please specify how you are dragging the scrollbar - mouse or touch?

Comment 2

[ovidiu boca[:Ovidiu]]

Sorry for missing that, I tested with mouse and touch and the actual result is the same.

Comment 3

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

I reproduced this and this is actually expected behaviour, because the test page intentionally blocks the main thread. While the main thread is blocked it cannot repaint, so that's why the text is delayed. The point of that test page is to demonstrate scrolling in the compositor while the main thread is blocked, and that's exactly what is happening. If apz.drag.enabled is turned off then dragging the scrollbar becomes super janky.

Closing as invalid since this is by design.

Comment 4

[ovidiu boca[:Ovidiu]]

I reproduce this behavior with another page: https://archive.mozilla.org/pub/firefox/tinderbox-builds/mozilla-inbound-linux-debug/1482891457/mozilla-inbound_ubuntu32_vm-debug_test-gtest-bm07-tests1-linux32-build277.txt.gz

I also saved that page locally and reproduce the same behavior, if I disable the apz.drag.enabled the problem is no longer reproducible. 

Please tell me your opinion about this, thanks.

Comment 5

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

What I see on that page looks like regular checkerboarding. It's mostly expected, but we are using bug 1251617 to track that issue for cases where it gets quite bad.

Comment 6

[ovidiu boca[:Ovidiu]]

I will add this test case to the bug 1251617 in order to verify this issue when the bug is fixed.  I also so that bug 1256677 is filed for tracking the checkerboarding issues, my question here is where should I add this test case, on which bug? Thanks

Comment 7

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

I would say bug 1251617 since your case is more targeted at testing the scrollbar dragging feature. There might be optimizations we can do specifically to that feature. Bug 1256677 is for checkerboarding using all scrolling methods and is more general.