668502 - scrolling tree scrolls container a bit the first time

Status: RESOLVED WONTFIX

(Core :: DOM: UI Events & Focus Handling, defect)

Description

[Neil Deakin]

Attachment: testcase

Steps:

1. Make sure the patch for bug 655004 is applied.
2. Open the testcase and mousescroll (I'm using the two-finger scroll on the mac trackpad) over the tree.
3. The tree scrolls but the container also scrolls a couple of pixels.

This issue only occurs the first time scrolling is done. It sometimes doesn't happen at all though.

It seems that the oddity relates to the nsEventStateManager::mLastLineScrollConsumedY field. It is a value that, if set, cancels the event. It defaults to false, but is set to true after the first scroll, causing different behaviour -- the event is cancelled/consumed before the event even gets to the node at http://mxr.mozilla.org/mozilla-central/source/content/events/src/nsEventStateManager.cpp#1308

I'm unclear what this code is trying to accomplish. It seems broken.

Comment 1

[Markus Stange [:mstange]]

If the tree wants to preventDefault all scroll events, then it also needs to call preventDefault on MozMousePixelScroll events, not only on DOMMouseScroll events.

The first DOMMouseScroll event of a scroll motion is only sent after a small threshold. If this threshold didn't exist, touchpad tree scrolling would probably feel more erratic because it'd be easier to trigger.

Comment 2

[Markus Stange [:mstange]]

Did that answer the question? Should I elaborate more?

And if you have a better idea of how to deal with the problem, I'm all ears :)

Comment 3

[Neil Deakin]

I'm not really clear what the exact meaning or difference between these two events is. Is there some documentation on this somewhere?

Comment 4

[Markus Stange [:mstange]]

There's https://developer.mozilla.org/en/Gecko-Specific_DOM_Events#MozMousePixelScroll and the code comment it links to.

MozMousePixelScroll is sent for every pixel scroll, i.e. it's the most fine-grained touchpad scroll event. DOMMouseScroll is only sent every several pixels (maybe about 15) for things that want to scroll row-wise instead of pixel-wise, for example trees. If a tree scrolled one row for every touchpad MozMousePixelScroll event, scrolling would be way too fast.

Comment 5

[Markus Stange [:mstange]]

For the complete, long-winded and repetitive discussion see bug 350471.