Instead of defining somewhat high level mouse actions, allow granular
control of mouse clicks and mouse down/up/move events. We will want to
simulate things like holding down a mouse button after double clicking
and then dragging the mouse to another position in the future, and this
enables that.
The flake was reproducible by running the entire LibWeb test suite over
and over again with sanitizers enabled. By making the test async and run
at window `load` time instead of document `DOMContentLoaded` time, I've
not been able to reproduce the issue locally.
Ideally I was able to find the underlying cause for this bug, but for
now I'd rather run this regression test.
We generated `PaintableFragment`s with a start and length represented in
UTF-8 byte offsets, but failed to consider that the offsets in a
`DOM::Range` are actually expressed in UTF-16 code units.
This is a bit of a mess: almost all web specs use UTF-16 code units as
the unit for indexing into text nodes, but we almost exclusively use
UTF-8 in our code base. Arguably the best thing would for us to use
UTF-16 everywhere as well: it prevents these mismatches in our
implementations for the price of a bit more memory usage - and even that
could potentially be optimized for.
But for now, try to do the correct thing and lazily allocate UTF-16 data
in a `PaintableFragment` whenever we need to index into it or if we're
asked to determine the code unit offset of a pixel position.
