[Webkit-unassigned] [Bug 191322] New: [css-position-sticky] miscalculating gesture to reach true end of scrollHeight when using sticky and negative margins

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=191322

            Bug ID: 191322
           Summary: [css-position-sticky] miscalculating gesture to reach
                    true end of scrollHeight when using sticky and
                    negative margins
           Product: WebKit
           Version: Safari Technology Preview
          Hardware: Macintosh
                OS: macOS 10.12
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: CSS
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: hi at jonjohnjohnson.com

Firstly, I'm sorry that this isn't as reduced of a test case as I, or probably you guys, would like, but I've struggled to isolate it apart from using negative margins of large/viewport units in conjunction with the sticky positioning scheme...

cc wenson_hsieh at apple.com

1. Go to test case -> https://jsbin.com/votodew/edit?css,output in large enough screen see it's background as white.
2. Scroll down through the red bordered "stacking/sticking" dialogue, reaching the very bottom.
3. Repeat the action of scrolling away from the bottom edge of the scrollHeight.
4. Notice if you sufficiently throw a scrolling gesture to reach scrolling end, but not with excessive force, more often than not, the final resting scroll position, will leave the scrollTop just short of reaching the end of the scroll container.
*Note, the easy visual that notes it not scrolling to the end is that the very last line of the "dialogue" is not aligned with the rest of the text, resting extra pixels below, depending on the viewport height.
5. When it "falls short" you can start another scrolling downward gesture that will reach it's end. But quite often it's miscalculating.

Problem
It seems as though mid scroll action, a sufficient gesture for reaching the calculated "scroll end" is shorter than the elements scrollHeight.

Expectation
Scrolling that "feels" as though it should reach the end of the scrolling container should not fall *just short of reaching the end.

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