1520121 - Confusion between logical and absolute scroll position for session history?

Status: NEW

(Core :: Layout: Scrolling and Overflow, enhancement, P3)

Description

[Jan Henning [:JanH]]

nsGfxScrollFrame normally uses GetLogicalVisualViewportOffset() when retrieving the current scroll position while dealing with session history.
Conversely, when restoring the scroll position we're transforming it back into an absolute one [1] before calling scrollTo().

There is one exception, though: If we're currently in a smooth scroll, the save logic just stores mDestination directly without turning it into a logical scroll position [2]. I suspect that this isn't correct.

[1] https://searchfox.org/mozilla-central/rev/b29663c6c9c61b0bf29e8add490cbd6bad293a67/layout/generic/nsGfxScrollFrame.cpp#4339
[2] https://searchfox.org/mozilla-central/rev/b29663c6c9c61b0bf29e8add490cbd6bad293a67/layout/generic/nsGfxScrollFrame.cpp#6149

Comment 1

[Jonathan Watt [:jwatt]]

And thoughts, Botond?

Comment 2

[Botond Ballo [:botond] [away until July 9]]

Jan's assessment seems accurate to me. It's likely an oversight.

One technique to avoid issues like this, that we've used with success elsewhere, is to represent logical vs. physical scroll offsets as different types. We have infrastructure to support this with out PointTyped and IntPointTyped classes [1], which allow encoding a "label" (Units) in the type of a point.

[1] https://searchfox.org/mozilla-central/rev/5c8ea961d04767db723a0a15e3a8f7fbca154129/gfx/2d/Point.h#123