When clicking on a glyph or starting a selection on it, we would use the
glyph's offset/index as the position which represents the left side of
the glyph, or the position between the glyph and the glyph before it.
Instead of looking at which glyph is under the mouse pointer, look at
which glyph boundary is closer. Now, if you click to the right of a
glyph (but still on that glyph), it correctly selects the next glyph's
offset as the position.
For example, in the following abbreviated test HTML:
<span>some text</span>
<script>println("whf")</script>
We would have to craft the expectation file to include the "some text"
segment, usually with some leading whitespace. This is a bit annoying,
and makes it difficult to manually craft expectation files.
So instead of comparing the expectation against the entire DOM inner
text, we now send the inner text of just the <pre> element containing
the test output when we invoke `internals.signalTextTestIsDone`.
In general it is not safe to convert any arbitrary floating-point value
to CSSPixels. CSSPixels has a resolution of 0.015625, which for small
values (e.g. scale factors between 0 and 1), can produce bad results
if converted to CSSPixels then scaled back up. In the worst case values
can underflow to zero and produce incorrect results.
