Change GetThebesLayerResolutionForFrame to GetThebesLayerScaleForFrame,
which just returns a scale. Ensure that the scale is as accurate as possible
even if dedicated layers for scrolled content (or any layers at all) have not
been created yet, by taking into account transforms that have not yet
generated layers. This makes the decisions made by
nsGfxScrollFrameInner::ScrollToImpl independent of whether there is
currently an active layer for the scrolled content (or much more nearly so).
In nsGfxScrollFrameInner::ScrollToImpl, do not use the current internal
fractional offset of the ThebesLayer, which is in a mostly unrelated
coordinate space to our scroll positions. Instead, just try to make sure
that the previous and next scroll position differ by a whole number of
layer pixels.
This means that font size inflation will be consistent into a container
with overflow!=visible, which fixes inflation bugs on phoronix.com (bug
747720) and reddit.com (bug 748446, but currently covered up by bug
756518).
Force building layers for all scroll frames that have a displayport set.
This fixes gfx/layers/ipc/CompositorParent mistakenly using FrameMetrics from
incorrect frames, due to the root scroll frame not having a layer.
If the current scroll position is fractional, e.g. y=N.4 CSS pixels, and something tries to read the
position in CSS pixels and scroll to that position (e.g. calling window.scrollTo(0, rootElem.scrollTop),
or equivalently window.scrollBy(0, 0)), it can actually end up scrolling backwards. So create a new
method nsIScrollableFrame::ScrollToCSSPixels which ensures that scrolling to a CSS pixel offset tries to
preserve the current fractional scroll position if that's possible, and if that's not possible at least does
not allow the scroll position to move in the wrong direction.
There was a subtle reftest failure on Mac in RTL text-overflow tests.
I tracked it down to RTL overflow:auto areas being automatically scrolled by a subpixel amount.
This was because we try to set scrollbar "curpos" attributes to an integer number of CSS pixels representing
the scroll amount, relative to the top-left of GetScrolledRect, which is a noninteger number of CSS pixels
in this case. Then in ReflowFinished() we scroll to the saved curpos values plus the top-left of GetScrolledRect.
This patch fixes the problem by making CurPosAttributeChanged pass an allowed scroll range to
ScrollToWithOrigin. We allow any scroll destination that, when rounded to CSS pixels, would give the same
value as "curpos".
This fixes the bug, ensuring that ReflowFinished's call to CurPosAttributeChanged will not normally need
to scroll because the current position will be in the acceptable range. Also, it means that code that
scrolls by setting the "curpos" attribute will be optimized to try to hit a layer pixel boundary.
