[CSS-UI] caret-shape: block/underscore and overflow

[schenney-chromium]

The spec for caret-shape sensibly says that a caret at the end of a text run should paint to the right (in ltr, left in rtl) where the next character in a run would appear. But what should happen if there is no space to put the caret in the container at that location?

Options include:

1. Clip the caret.
2. Start a new line and put the caret on that line.
3. Paint the caret as close as possible to the edge, even if it overlaps the last character in the run.
4. Treat the caret as overflow and apply the overflow rules for the container.

Terminal programs on Mac and Linux, the things I can easily check, both exhibit behavior (2). There is no existing browser implementation other than maybe Opera (according to canIuse).

Igalia is considering a chromium implementation and this is a question that immediately comes to mind as a blocker.