While layers are active, scale factors are normalized to a power of 2 to
reduce the number of resolution changes during animations. This patch disables
the normalization when the scale factor on the layer is unchanged, or when
the layer was previously unscaled.
This lets us avoid clamping the scale in more situations. We should only clamp the scale when we think
the scale is changing due to a changing transform --- the goal of clamping is to not have to redraw the content
too often when the content is zooming in or out.
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.
The basic idea is that whenever a layer transaction updates the window, we clear out the invalidation state for the canvas rendering context,
using a DidTransactionCallback registered on the layer(s) for the canvas, which calls MakeContextClean.
The DidTransactionCallbacks are directed to the user data attached to the Layer, which holds a strong reference to the canvas element. This
ensures that the element lives as long as the layer. Layers are destroyed when the presentation is torn down (including if the frame is destroyed),
so we can't have a leak here. The reference to the canvas element is only strong because the layer might briefly outlive the frame (the layer
won't be destroyed until the next paint of the window).
This patch moves responsibility for calling CanvasLayer::Updated and nsFrame::MarkLayersActive from the canvas context to nsHTMLCanvasElement::InvalidateFrame.
We call Updated on the retained CanvasLayer, if there is one; any other CanvasLayers created for this canvas would only be used once, and have Updated
called on them in BuildLayer when created.
