Nu properly marks input areas with OSC 133 B, but if it spans multiple
lines it doesn't mark the continuation lines with the k=s sequence.
Our prompt redraw logic before only cleared explicitly designated prompt
lines. But if the input line is multi-line and the continuation lines
are not marked, those lines would not be cleared, leading to visual
issues on resize.
To workaround this, we assume that if the current cursor semantic
content is anything other than "command output" (default), then we're
probably at a prompt line and should clear from there all the way up.
This works around Fish (at least v4.2) having a non-compliant OSC133
implementation paired with not having the hooks to fix this via shell
integration. We have to instead resort to heuristics in the terminal
emulator. Womp, womp.
The issue is that Fish does not emit OSC133 secondary prompt (`k=s`)
markers at the beginning of continuation lines. And, since Fish doesn't
provide a PS2-equivalent, we can't do this via shell integration.
We fix this by assuming on newline (`\n`) that a cursor that is already
painting prompt cells is continuing a prior prompt line, and
pre-emptively mark it as a prompt line. But this has two further issues
we have to work around:
1. Newline/index (`\n`) is one of the _hottest path_ functions in
terminal emulation. It sucks to add any new conditional logic here.
We do our best to gate this on unlikely conditions that the branch
predictor can easily optimize away.
2. Fish also emits these for auto-complete hints that may be deleted
later. So, we also have to handle the scenario where a prompt is
continued, then replaced by command output, and fix up the prompt
continuation flag to go back to output mode.
Point 2 is ALMOST automatically handled, because Fish does emit a `CSI J`
(erase display below) to erase the auto-complete hint. This resets all
our rows back to output rows. **Unfortunately**, Fish emits `\n` before
triggering the preexec hooks which set OSC133C. So we get the newline
logic FIRST (sets the prompt line), THEN sets the output cursor. If they
switched ordering here everything would just work (with the one
heuristic). But now, we need two!
To address this, I put some extra heuristic logic in the OSC133C
(output starting) handler: if our row is marked as a prompt AND our
cursor is at x=0, we assume that the prompt continuation was deleted
and we unmark it.
I put the heuristic logic dependent on OSC133C because that's way colder
of a path than putting something in `printCell` (which is the actual
hottest path in Ghostty).
We could get more rigorous here by also checking if every cell is empty
but that doesn't seem to be necessary at this time for any Fish version
I've tested. I hope thats correct.
I'd really love for Fish to improve their OSC133 implementation to
conform more closely to the terminal-wg spec, but we're going to need
these workarounds indefinitely to handle older Fish versions anyway.
Insert OSC 133 A k=s marks after each newline in PS1, so that all lines
following the first are marked as secondary prompts. This prevents ghostty
from erasing leading lines during terminal resize.
