A comment in ApplyThebesLayerInvalidation says that it preserves the content
of ThebesLayerInvalidRegion, in case there are multiple container layers for
the same frame. SetHasContainerLayer, however, immediately clears said property.
This was causing invalidations to be lost since Bug 758620 on fixed-position
elements, as they were being separated out onto their own layers but were still
merged in the root scroll layer. This is tracked in Bug 769541.
This fixes the problem by storing the new invalid region in DisplayItemDataEntry
and clearing/setting the ThebesLayerInvalidRegion property in the
UpdateDisplayItemData callback from FrameLayerBuilder::WillEndTransaction.
Previously we snapped the results of nsDisplayItem::GetBounds and
nsDisplayItem::GetOpaqueRegion internally. By tracking which display items were
inside transforms, we disabled snapping quite conservatively whenever an ancestor
had a transform, which is undesirable.
With this patch, we don't snap inside GetBounds or GetOpaqueRegion, but just return
a boolean flag indicating whether the item will draw with snapping or not. This flag
is conservative so that "true" means we will snap (if the graphics context has a transform
that allows snapping), but "false" means we might or might not snap (so it's always safe
to return false).
FrameLayerBuilder takes over responsibility for snapping item bounds. When it converts
display item bounds to layer pixel coordinates, it checks the snap flag returned from
the display item and checks whether the transform when we draw into the layer will be
a known scale (the ContainerParameters scale factors) plus integer translation. If both
are true, we snap the item bounds when converting to layer pixel coordinates. With
this approach, we can snap item bounds even when the items have ancestors with active
transforms.
FrameLayerBuilder::BuildContainerLayerFor takes responsibility for resolution scaling. The ContainerParameters
passed in are added to any transform requested. Then we extract the scale part of the transform, round the scale
up to the nearest power of two if the transform may be actively animated (so we don't have to redraw layer contents
constantly), pass that scale down to be applied by each child and set the residual transform on the ContainerLayer.
For child layers built via BuildLayer, we just pass the requested scale factor in via the ContainerParameters.
If the returned layer is a ContainerLayer then BuildLayer is guaranteed to have already done necessary scaling.
If the returned layer is not a ContainerLayer then we apply the scale ourselves by adding the scale to the
child layer's transform.
For child ThebesLayers containing non-layer display items, we scale the drawing of those display items so that
the child ThebesLayers are simply larger or smaller (larger or smaller visible regions).
We have to scale all visible rects, clip rects etc that are in the coordinates of ThebesLayers or the parent
ContainerLayer. To keep things simple we do this whenever we convert from appunits to integer layer coordinates.
When a ThebesLayer's resolution changes we need to rerender the whole thing.
nsDisplayList::PaintForFrame needs to respect the presshell's resolution setting. We do that by building a layer tree
with a ContainerParameters requesting a scale up by the presshell resolution; once that layer tree is built, we
adjust the root layer transform to scale back down by the resolution.
This patch shouldn't change any behavior. It just passes the ContainerParameters around, which will contain scale factors that should have been
applied when BuildLayer returns a ContainerLayer.
This patch also adds an aTransform parameter to BuildContainerLayerFor, which nsDisplayTransform uses to set the
transform for the ContainerLayer. This way BuildContainerLayerFor knows what the container's transform is going to be
before constructing the children, which in the next patch will let us construct the children with the right resolution.
