References <https://bugzilla.mozilla.org/show_bug.cgi?id=1879559>
(“In HCM, the text selection is barely visible”).
Continues work from @calixteman who had a partial patch.
This PR improves viewer text-selection highlighting by rendering
selection shapes in the draw layer.
* add selection overlay rendering in the draw layer
* significant code relates to selections spanning multiple text
layers/pages, and edges/end-of-content boundaries
* clear selection on rotate/scale/scroll/spread changes
My main question is: how should it appear?
I don’t have access to the Figma file linked on bugzilla.
In the CSS (`draw_layer-builder.css`) there are 3 blocks:
* default
* `@supports` for browsers supporting `backdrop-filter`
* `forced-colors` mode
So it’s possible to design for those (or more).
Personally, the `backdrop-filter: invert(1)` is the most contrast,
so perhaps it’s better to use something else as the default,
and to use `invert(1)` if high contrast mode is used (maybe with a
`prefers-contrast` media query instead)?
The goal is to be able to get these outlines to fill the shape corresponding
to a text selection in order to highlight some text contents.
The outlines will be used either to show selected/hovered highlights.
