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Add support for selective erase operations
This PR adds support for the selective erase escape sequences: DECSED,
DECSEL, and DECSCA. They provide a way of marking certain areas of
the screen as "protected", so you can erase the content everywhere else
without affecting those protected areas.
This adds another bit in the CharacterAttributes enum to track the
protected status of each cell, and an operation triggered by the
DECSCA sequence which can toggle that bit in the active attributes.
There there are two new erase operations triggered by the DECSED and
DECSEL sequences, which work similar to the existing ED (erase in
display) and EL (erase in line) operations, but which only apply to
unprotected cells.
I've also updated the DECRQSS settings request, so you can query the
active protected attribute status.
Validation Steps Performed
I've manually confirmed that we pass the selective erase tests in Vttest now, and I've also manually tested some more complicated edge cases and confirmed that we match the behavior of the hardware VT240 emulator in MAME.
For unit testing I've extended the existing erase tests to cover
selective erase as an additional option, I've added a test covering the
DECSCA sequence, and I've extended the DECRQSS adapter test to
confirm the attribute reporting is working.
Closes #14029
Also noted: part of making it transit ConPTY (if we were so inclined) would be to make triply sure the (ever-problematic) optimization to avoid sending empty spaces didn't optimize out a protection attribute :)
Also noted: part of making it transit ConPTY (if we were so inclined) would be to make triply sure the (ever-problematic) optimization to avoid sending empty spaces didn't optimize out a protection attribute :)
The protected attributes don't need to pass over conpty, because all of the logic is happening on the conhost side (edit: I see now you already noted that in the comment above).
Although now that I think about it, it's possible the existence of a protected attribute might prevent spaces being optimized where they potentially could have been, but that shouldn't break anything. I have tested this in Windows Terminal, and haven't seen any problems.
Edit: Looking at the HasIdenticalVisualRepresentationForBlankSpace implementation here:
https://github.com/microsoft/terminal/blob/1ce22a87e6fc204b5fd98642723aa0a51a503749/src/buffer/out/TextAttribute.hpp#L143-L152
Technically that _attrs == other._attrs test could potentially mask out the protected attribute, but I'm not sure I want to introduce any additional complexity here to optimise what is likely an uncommon edge case.
Hello @DHowett!
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I'm assuming this is ready to merge.
@msftbot merge this in 10 minutes
EDIT: huh, looks like the bot isn't working? Merging!
:tada:Windows Terminal Preview v1.17.1023 has been released which incorporates this pull request.:tada:
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