With APZ, we always layerize the first scrollable element of the page,
if the page itself is not scrollable. These additional layers can cause
fuzzy reftest failures in two ways: differences in blending, and by
disabling sub-pixel test anti-aliasing. The latter is only a problem
with unaccelerated drawing, because we don't support component alpha
layers with BasicLayers. (We also don't support them with
BasicCompositor, but "Reftest unaccelerated" only tests BasicLayers).
This patch adds a change hint to signal that one of the offsets on a
frame has been changed. When processing the hint, we do one of the
following things based on the position property of the frame.
* For static frames, we ignore the offset changes completely, as they
will not change the layout.
* For relative positioned frames, this patch refactors the
nsHTMLReflowState::ComputeRelativeOffsets function so that it can be
used from other callers, and it uses that to compute the new relative
offsets, and uses the offsets computed previously to compute the new
position of the frame.
* For absolute positioned frames, we set up a fake parent reflow state
object, and then we create a new reflow state object for the frame in
question. This setup is similar to what nsFrame::BoxReflow does.
Once we have the new reflow state object, we use it to compute the
absolute offsets, and then we use the computed offsets to set the new
position of the frame. The offset computation is similar to what
nsAbsoluteContainingBlock::ReflowAbsoluteFrame does. In some cases
where it is possible for the dimensions of the frame to change based
on the offset changes, we fall back to a full reflow.
Kartikaya Gupta
Boris Zbarsky
David Anderson
Phil Ringnalda
Alexandre Lissy
Jonathan Kew
CJKu
Andrew Halberstadt
Vincent Chen
Robert O'Callahan
Corey Ford
Matt Woodrow
Ehsan Akhgari
Ryan VanderMeulen