There are scenarios where this configuration looks bad. This commit
introduces some heuristics to prevent it. Here are the heuristics:
* Extension is always enabled on alt screen.
* Extension is disabled if a row contains any default bg color. The
thinking is that in this scenario, using the default bg color looks
just fine.
* Extension is disabled if a row is marked as a prompt (using semantic
prompt sequences). The thinking here is that prompts often contain
perfect fit glyphs such as Powerline glyphs and those look bad when
extended.
This introduces some CPU cost to the extension feature but it should be
minimal and respects dirty tracking. This is unfortunate but the feature
makes many terminal scenarios look much better and the performance cost
is minimal so I believe it is worth it.
Further heuristics are likely warranted but this should be a good
starting set.
Some CoreFoundation objects, such as those produced by CoreText, have
expensive callbacks that run when they are released. By offloading the
CFRelease calls to another thread, we can avoid important threads being
blocked by unexpectedly expensive callbacks.
This commit also changes the way that the coretext shaper's run iterator
builds its string. Rather than using a CFMutableString, an ArrayList of
unichars is built which is passed to CFStringCreateWithCharactersNoCopy,
which is a lot more efficient since it avoids all the CoreFoundation
overhead.
Related to #1768 but doesn't fix it properly.
This is a temporary hack to avoid some issues with fonts that have mixed
color/non-color glyphs. If there are mixed presentations and the font
does not have emoji codepoints, then we assume it is text. This fixes
the typical scenarios.
This is not a long term solution. A proper long term solution is to
detect this scenario and on a per-glyph basis handle colorization (or
the lack thereof) correctly. It looks like to do this we'll have to
parse some font tables which is considerably more work so I wanted to do
this first.
