VT renderer works incorrectly when cursor is moved between writes about terminal HOT 10 CLOSED

MRE: VtRendererBug.zip
STR: compile, run, follow instructions

P.S. I think the provided example reproduces the same issue.
Just in case if not, STR for the original one:

1. Get Far - https://github.com/FarGroup/FarManager/releases/
2. Run it.
3. In F9 - Options - Interface settings
   uncheck:
   [ ] Use Virtual Terminal for rendering
[ ] Fullwidth-aware rendering
   check:
   [x] ClearType-friendly redraw (can be slow)
4. Edit any file (F4)
5. In F11 - FarColorer - Configure - Main settings
   check:
   [x] Cross
   set Cross style to both
6. Press CtrlHome to move the cursor to the upper left corner and then Right a few times.

from terminal.

I've just confirmed that reverting commit 72b4488 on the main branch is enough to fix the issue for me (i.e. moving the WaitUntilCanRender call back down to the bottom of the loop, before PaintFrame). And I've also now tested the cursor movement in Far itself, and the same fix works there too.

Again I'm not sure that's a real solution to the problem, but it might be worth considering as a quick fix, assuming it works for everyone else and not just me.

from terminal.

As for what's going wrong, this is what I've been able to establish:

1. When you write a line of text that extends to the last column, we set the _wrapForced flag, even though the line hasn't actually wrapped (this is partially tracked in #15602, although that's referencing the AdaptDispatch class, while this particular case is in one of the console APIs).

2. When the VtEngine outputs that line, and it sees that flag is set, it records the apparent wrapped state in the _wrappedRow field, again despite the fact that it hasn't actually wrapped.

3. At this point in time, the cursor is meant to be at the start of line 2, because that's where it was manually positioned, and the WriteConsoleOutput API doesn't change that. So at the end of the frame, the VtEngine tries to move the cursor there. However, because of _wrappedRow being set to the line above, it thinks there is nothing to do (the previousLineWrapped test here).

4. Even though the _MoveCursor method didn't do anything in the previous step, it still updates the _lastText field to where it mistakenly thought the cursor was (it thinks it's at the start of line 2, when it's actually at the end of line 1). This is why the subsequent output ends up the wrong location.

Ignoring bug number 1 for the moment, it seems to me that point 3 is also a bug, because the _wrappedRow field being set does not actually indicate that the cursor is on the next line, only that it doesn't need to be moved there if you're about to write a character. If you're just painting the cursor at that point, it needs to be moved explicitly.

So another possible way to fix this issue would be to set _wrappedRow = std::nullopt in the VtEngine::PaintCursor method before calling _MoveCursor.

That said, the conpty code is complicated, and I really don't know it very well, so there may be more to it than that. I'm just throwing this out there as potential solution.

from terminal.

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Closed similar issues:

• Issues with cursor invalidation in GDI (#17150), similarity score: 0.75

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from terminal.

We've talked internally about reverting #15500 for the 1.21 release. It's the source for all these issues. While the fix you found may fix this particular issue, it's not unlikely that there will be more.

On the current main branch, which targets version 1.22, we're thinking of potentially removing VtEngine in its entirety. This would then also fix the cursor invalidation issue introduced in #15500. Unfortunately, this means that this issue would intentionally exist on main for up to a couple months.

from terminal.

FYI, I did a git bisect with the VtRendererBug test case, and for me the issue first showed up in commit 72b4488. However, I suspect it may be one of those things that's timing dependent, and that commit just exposed a bug that has always existed.

from terminal.

Thanks James, reverting 72b4488 fixed it for me as well.

from terminal.

Another simple test case that can be run from a WSL bash shell:

printf '\033[2J\033[1;999H*'; sleep 1; printf '\033[2H\033[65;1;1;1;999$x'; sleep 1; printf '\033[3HThis should be line 3\n'

The text "This should be line 3", should be on line 3 😄, but it usually ends up on line 2.

This one is not fixed by reverting 72b4488, but it is fixed by the _wrappedRow patch suggested above.

from terminal.

Whichever the fix ends up being, I'm going to defer it until the next servicing wave for 1.21 - thanks for all the great investigations, everyone 😄

from terminal.

I've just realised that issue #17013 is another variant of this bug. And that scenario is also fixed by PR #17290.

from terminal.