Terminal: linewrap then backspace produces unexpected visuals, missing characters

[fredless]

Environment

Windows build number: Microsoft Windows [Version 10.0.18362.175]
Windows Terminal version (if applicable): built using d82eab44d076e441f0b630d9b53a19dfa884c3ab

Steps to reproduce

• open terminal with following profile:

{
    "globals" : 
    {
        "alwaysShowTabs" : false,
        "defaultProfile" : "{9d2406b9-7e40-4e2b-a20d-82dc61c4cc24}",
        "initialCols" : 80,
        "initialRows" : 60,
        "requestedTheme" : "dark",
        "showTabsInTitlebar" : true,
        "showTerminalTitleInTitlebar" : false
    },
    "profiles" : 
    [
        {
            "acrylicOpacity" : 0.80000001192092896,
            "closeOnExit" : true,
            "colorScheme" : "Solarized Dark",
            "commandline" : "cmd.exe",
            "cursorColor" : "#FFFFFF",
            "cursorShape" : "bar",
            "fontFace" : "Consolas",
            "fontSize" : 10,
            "guid" : "{9d2406b9-7e40-4e2b-a20d-82dc61c4cc24}",
            "historySize" : 9001,
            "name" : "cmd",
            "padding" : "0, 0, 0, 0",
            "snapOnInput" : true,
            "startingDirectory" : "%USERPROFILE%",
            "useAcrylic" : true
        }
    ]
}

• input characters until the line wraps, backspace once, and enter an additional character

Expected behavior

• Cursor should return to the previous position following the last character on the previous line after hitting backspace.
• Additional input character (sent after backspace) should show up after the last character on the previous line.

Actual behavior

• Cursor returns to previous line, but with a whitespace between the last character input and the cursor position
• Additional input character does not show up in terminal, although it is recognized

'..789ab' is received as input in the example below, even though it is not shown:

[DHowett-MSFT]

This is probably something down in the pseudoconsole layer; we probably have a duplicate for it, but we can't find it.

[fredless]

Should have also mentioned in case it may be helpful, identical behavior was observed on 73ad742c12c0d86a6edd4982ab7ee5590684d27e

Didn't report then as that was my first exposure, assumed either I couldn't be the only one (or that I was). I just repulled and built in hopes it might have been squashed. I searched then and now before submitting but didn't see a specific match.

[fredless]

I can't be the only one.. can I?

[zadjii-msft]

@fredless you're not the only one - this is a reasonable bug report with clear repro steps. Doesn't seem like there's much that needs to be added until someone starts working on a fix.

[zadjii-msft]

After playing with this some more, this looks like it _only_ repros in the Terminal. This might actually just be another variant of WriteCharsLegacy2ElectricBoogaloo. I doubt we're putting the cursor in the right spot when we backspace around a linewrap like this.

cc @miniksa.

[miniksa]

Backspace around a line is a WriteCharsLegacy handled thing in the conhost. So your reasoning is sound. This should be linked to #780. Maybe not resolved until we're sure that it will resolve this too.

[riverar]

Still seeing this issue in 0.8.10091.0.

[DHowett-MSFT]

Thank you for the report. If we ever think we have explicitly fixed this bug, we’ll be sure to close this issue.

[zadjii-msft]

For the record, I've merged a combo of branches for #4354, #4289, and dev/migrie/f/conpty-wrapping-003 all together, and this still repros. It _only_ repros in cmd.exe, and not bash however. I'm thinking it's possible that it's related shockingly to another bug where the cursor doesn't get moved by conpty on a frame where only the cursor changed position - maybe some combo of #4102/#3202. I'll keep digging.

[zadjii-msft]

Hokay I've got a fix for this that I'm a little wary of, but I'm not sure it can be de-tangled from dev/migrie/f/conpty-wrapping-003. I'll investigate.

[zadjii-msft]

@\:

This is the other side of the #357 coin actually.

Changing the fix for #357 from
https://github.com/microsoft/terminal/blob/814f4caf3eb3e44c37d3c35bb318423a5ef54a5d/src/renderer/vt/paint.cpp#L452-L467

to

    if (_lastText.X < _lastViewport.RightExclusive())
    {
        _lastText.X += static_cast<short>(columnsActual);
    }

fixes the Terminal, but re-breaks using conhost as the Terminal (interop/vtpipeterm).

Maybe the Terminal is _just wrong_ here (wouldn't surprise me). Especially considering this bug doesn't repro in conhost as a terminal. Perhaps this is delayed eol wrap? I'll look at the traces, see what's going on.

[msftbot[bot]]

:tada:This issue was addressed in #4403, which has now been successfully released as Windows Terminal Preview v0.9.433.0.:tada:

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