We only checked if the paintable box had scrollable overflow, but that
was too simple - we now use the same logic that checks whether a box can
be scrolled by a mousewheel event.
The autoscrolling test was updated as well to use rAF+rAF+timeout
instead of a fixed 1000ms timeout, which is prone to flakiness.
Replace per-node heap-allocated AtomicRefCounted
AccumulatedVisualContext objects with a single contiguous Vector inside
AccumulatedVisualContextTree. All nodes for a frame are now stored in
one allocation, using type-safe VisualContextIndex instead of RefPtr
pointers.
This reduces allocation churn, improves cache locality, and opens the
door for future snapshotting of visual context state — similar to how
scroll offsets are snapshotted today.
Extract the repeated pattern of transforming a rectangle from absolute
coordinates to viewport coordinates via the accumulated visual context
into a helper method.
This was arguably put in a worse place by #8162; we mostly need the
device pixel offsets from the scroll state so keep track of those and
convert back to CSS pixels when necessary (i.e. scrollbar data).
Stop converting between CSS and device pixels as part of rendering - the
display list should be as simple as possible, so convert to DevicePixels
once when constructing the display list.
We were using a separately fired timer for auto scrolling ticks, but it
makes more sense to tie this into the rendering steps of which
`::run_the_scroll_steps()` is a part. Should fix the flaky
`Text/input/viewport-auto-scroll.html` test.
Fixes#7939.
When the mouse is dragged from inside a scrollable container to outside
of it, we now automatically scroll the container so the selection can be
extended. Scroll speed scales with the distance past the scrollport
edge, capped at a maximum. Edges close to the viewport boundary get a
wider activation zone so the speed ramp works predictably even when the
mouse has limited room to move.
The logic is encapsulated in AutoScrollHandler, which EventHandler
creates lazily on mouse selection start.
