Iris, correcting the previous commit: "Why does the mask matter at all.
If you want a mask then you add .masked(). It should just prevent rows
that aren't in its region at all from drawing ... Just like the opt in
scrollable, masking should be opt in."
So `LazySpan` sets no mask. It culls -- a row entirely outside the box it
was offered is never drawn, which `intersects_viewport` already did -- and
draws a straddling row in full, because virtualisation decides which rows
and never how much of one. Cutting off that overhang is `.masked()`, added
by whoever wants it.
The transcript wants it (it is a list under a header bar) and opts back
in; the benchmark does not and needs no ceremony. `top_edge.rs` goes back
to reading the mask the list *inherited*, which is now also the test that
the transcript is still asking for one.
The previous commit had the span mask itself, which fixes the panic and is
still the widget deciding what is not its to decide.
iris
e9a6562dc6