There are only 'skip'ped commits left to test.
The first bad commit could be any of:
1f3b5e1a826fbff8ce2fa84e0e50d082d65a08bd
6910bfee0fb9470b0b30e5a46efeebfe760e6a2c
We cannot bisect more!

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git bisect start
# bad: [27356de2697ca542ded05ea6c2017a3ca738084f] Merge pull request #7104 from jamessan/run-oldtests-under-headless
git bisect bad 27356de2697ca542ded05ea6c2017a3ca738084f
# good: [f0dafa89c2b7602cfedf0bd3409858e4c212b0a2] provider/clipboard.vim: Handle missing g:clipboard keys
git bisect good f0dafa89c2b7602cfedf0bd3409858e4c212b0a2
# bad: [105d680aea9f449c118520597a822e834a00c0ac] Merge #6816 'TUI improvements'
git bisect bad 105d680aea9f449c118520597a822e834a00c0ac
# bad: [cdfaecb25f6a9a94f29a38d9f2d24a579b3dff5f] tui: Eliminate more extraneous control sequences.
git bisect bad cdfaecb25f6a9a94f29a38d9f2d24a579b3dff5f
# skip: [5377de33ac4adea54c9c9980c7a6bc8cc70c0c7a] tui: Add st to the always 256-colour capable list. Also comment and augment some terminal colour tests.
git bisect skip 5377de33ac4adea54c9c9980c7a6bc8cc70c0c7a
# bad: [5265ac500064eecba3317ad8152cab14e9950db7] tui: Change terminal family recognition to avoid '+' entries. The terminfo commentary states that these are not standalone entries suitable for end-use.
git bisect bad 5265ac500064eecba3317ad8152cab14e9950db7
# good: [74472f7b2bf354dbffcaca3738dacba5dfaefd6d] tui: Fix conflict with predefined "linux" macro.
git bisect good 74472f7b2bf354dbffcaca3738dacba5dfaefd6d
# skip: [1f3b5e1a826fbff8ce2fa84e0e50d082d65a08bd] tui: Correct error in terminfo extension processing. Using the wrong unibilium query function just happened to work with the various terminfo records used in local testing.
git bisect skip 1f3b5e1a826fbff8ce2fa84e0e50d082d65a08bd
# good: [16300d02c7fbe95396697fc6c09012e2c2b47b69] tui: Improvements to RGB colour support. The details are in the on-line help under :help true-color .
git bisect good 16300d02c7fbe95396697fc6c09012e2c2b47b69
# bad: [6910bfee0fb9470b0b30e5a46efeebfe760e6a2c] tui: Correct error in terminfo extension processing. Using the right unibilium query function then revealed a latent NULL pointer problem.
git bisect bad 6910bfee0fb9470b0b30e5a46efeebfe760e6a2c
# good: [8768b7f4a045093001c55d12a85419bebe5869dc] tui: Remove now-unused flag variable. Follows on from fcf0d13f48bffbd41749069ce383d01153dd960c.
git bisect good 8768b7f4a045093001c55d12a85419bebe5869dc
# only skipped commits left to test
# possible first bad commit: [6910bfee0fb9470b0b30e5a46efeebfe760e6a2c] tui: Correct error in terminfo extension processing. Using the right unibilium query function then revealed a latent NULL pointer problem.
# possible first bad commit: [1f3b5e1a826fbff8ce2fa84e0e50d082d65a08bd] tui: Correct error in terminfo extension processing. Using the wrong unibilium query function just happened to work with the various terminfo records used in local testing.