Font inflation flow roots are similar to block formatting contexts,
though I'm trying to make not all block formatting contexts be flow
roots in later patches on this bug, bug 707195, and related bugs.
This will lead to more consistent font size inflation in a flow of text
where the blocks vary in width (e.g., because some of the blocks have
horizontal margins) but we determine the text to comprise a single flow.
The basic principle behind this preference is that we don't want to use
font size inflation when all the text that we would have inflated would
fit within a line or two. We're better off preserving the layout than
inflating a small amount of text. However, we don't want the inflation
decision to vary between adjacent paragraphs, so this decision needs to
be made once for each block formatting context.
Once the later patches for this bug cause this preference to be honored,
this will be a change in behavior. The old behavior can be obtained by
setting the preference to 0.
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.
This is the third of three patches to rework the way we handle getting
the font inflation container and width data during reflow, which are
needed so that we can sometimes honor inflation during intrinsic width
calculation (which we need to do to make some form controls inflate
correctly).
This is the first of two patches to honor inflation during intrinsic
width calculation (which we need to do to make some form controls
inflate correctly).
This moves the logic currently in
nsLayoutUtils::IsContainerForFontSizeInflation into frame initialization
so that we can (later, for bug 706193) make that logic more complicated.
