Introduces the rendering of scroll thumbs in vertical and horizontal
directions. Currently, the thumbs are purely graphical elements that
do not respond to mouse events. Nevertheless, this is beneficial as it
makes it easier to identify elements that should respond to scrolling
events.
Painting of scrollbars uncovers numerous bugs in the calculation of
scrollable overflow rectangles highlighting all the places where
elements are made scrollable whey they shouldn't be. Positively, this
issue might motivate us to pay more attention to this problem to
eliminate unnecessary scrollbars.
Currently, the scrollbar style is uniform across all platforms: a
semi-transparent gray rectangle with rounded corners.
Also here we add `scrollbar-width: none` to all existing scrolling
ref-tests, so they keep working with this change.
With this change, instead of applying scroll offsets during the
recording of the painting command list, we do the following:
1. Collect all boxes with scrollable overflow into a PaintContext,
each with an id and the total amount of scrolling offset accumulated
from ancestor scrollable boxes.
2. During the recording phase assign a corresponding scroll_frame_id to
each command that paints content within a scrollable box.
3. Before executing the recorded commands, translate each command that
has a scroll_frame_id by the accumulated scroll offset.
This approach has following advantages:
- Implementing nested scrollables becomes much simpler, as the
recording phase only requires the correct assignment of the nearest
scrollable's scroll_frame_id, while the accumulated offset from
ancestors is applied subsequently.
- The recording of painting commands is not tied to a specific offset
within scrollable boxes, which means in the future, it will be
possible to update the scrolling offset and repaint without the need
to re-record painting commands.
