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[PR #6695] Restore simple text runs, correct for crashes #26771

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opened 2026-01-31 09:18:04 +00:00 by claunia · 0 comments
claunia commented 2026-01-31 09:18:04 +00:00
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Original Pull Request: https://github.com/microsoft/terminal/pull/6695

State: closed
Merged: Yes

Restores the simple text run analysis and skipping of most of the
shaping/layout steps. Corrects one of the fast-path steps to ensure that
offsets and clusters are assigned.

References

The "correction" functions, by which box drawing analysis is one of
them, is dependent on the steps coming before it properly assigning the
four main vectors of the text layout glyphs: indices, advances, offsets,
and clusters. When the fast path is identified by the code from #6206,
only two of those are fully updated: indices and advances. The offsets
doesn't tend to cause a problem because offsets are rarely used so
they're pretty much always 0 already (but this PR enforces that they're
zero for the simple/fast path.) The clusters, however, were not mapped
for the fast path. This showed itself in one of two ways:

  1. Before the recycled runs PR #6483, the cluster map had a 0 in every
    field for the stock initialized vector.
  2. After the recycled runs PR #6483, the cluster map had the previous
    run's mapping in it.

This meant that when we reached the steps where glyph runs were
potentially split during the correction phase for box drawing
characters, unexpected values were present to map the glyph indices to
clusters and were corrected, adjusted, or split in an unexpected
fashion.

For instance, the index out of range bug could appear when the default 0
values appended to the end of the clusters vector were decremented down
to a negative value during the run splitter as the true DWrite cluster
mapper doesn't generate that sort of pattern in the slow path case
without also breaking the run itself.

The resolution here is therefore to ensure that all of the fields
related to glyph layout are populated even in the fast path. This
doesn't affect the slow path because that one always populated all
fields by asking DWrite to do it. The fast path just skips a bunch of
DWrite steps because it can implicitly identify patterns and save a
bunch of time.

I've also identified a few vectors that weren't cleared on reset/reuse
of the layout. I'm clearing those now so the .resize() operations
performed on them to get to the correct lengths will fill them with
fresh and empty values instead of hanging on to ones that may have been
from the previous. This should be OK memory perf wise because the act of
.clear() on a vector shouldn't free anything, just mark it invalid.
And doing .resize() from an empty one should just default construct
them into already allocated space (which ought to be super quick).

Validation

  • Far.exe doesn't crash and looks fine
  • "\e[30;47m\u{2500} What \u{2500}\e[m" from #6488 appears
    appropriately antialiased
  • Validate the "\e[30;47m\u{2500} What \u{2500}\e[m" still works
    when FillGeometry is nerfed as a quick test that the runs are split
    correctly.
  • Put `u{fffd} into Powershell Core to make a replacement char in
    the output. Then press enter a few times and see that shrunken initial
    characters on random rows. Verify this is gone.

Closes #6668
Closes #6669

Co-Authored-By: Chester Liu skyline75489@outlook.com

**Original Pull Request:** https://github.com/microsoft/terminal/pull/6695 **State:** closed **Merged:** Yes --- Restores the simple text run analysis and skipping of most of the shaping/layout steps. Corrects one of the fast-path steps to ensure that offsets and clusters are assigned. ## References - Bug #6488 - Bug #6664 - Simple run PR #6206 - Simple run revert PR #6665 - Recycle glyph runs PR #6483 The "correction" functions, by which box drawing analysis is one of them, is dependent on the steps coming before it properly assigning the four main vectors of the text layout glyphs: indices, advances, offsets, and clusters. When the fast path is identified by the code from #6206, only two of those are fully updated: indices and advances. The offsets doesn't tend to cause a problem because offsets are rarely used so they're pretty much always 0 already (but this PR enforces that they're zero for the simple/fast path.) The clusters, however, were not mapped for the fast path. This showed itself in one of two ways: 1. Before the recycled runs PR #6483, the cluster map had a 0 in every field for the stock initialized vector. 2. After the recycled runs PR #6483, the cluster map had the previous run's mapping in it. This meant that when we reached the steps where glyph runs were potentially split during the correction phase for box drawing characters, unexpected values were present to map the glyph indices to clusters and were corrected, adjusted, or split in an unexpected fashion. For instance, the index out of range bug could appear when the default 0 values appended to the end of the clusters vector were decremented down to a negative value during the run splitter as the true DWrite cluster mapper doesn't generate that sort of pattern in the slow path case without also breaking the run itself. The resolution here is therefore to ensure that all of the fields related to glyph layout are populated even in the fast path. This doesn't affect the slow path because that one always populated all fields by asking DWrite to do it. The fast path just skips a bunch of DWrite steps because it can implicitly identify patterns and save a bunch of time. I've also identified a few vectors that weren't cleared on reset/reuse of the layout. I'm clearing those now so the `.resize()` operations performed on them to get to the correct lengths will fill them with fresh and empty values instead of hanging on to ones that may have been from the previous. This should be OK memory perf wise because the act of `.clear()` on a vector shouldn't free anything, just mark it invalid. And doing `.resize()` from an empty one should just default construct them into already allocated space (which ought to be super quick). ## Validation * [x] Far.exe doesn't crash and looks fine * [x] "\e[30;47m\u{2500} What \u{2500}\e[m" from #6488 appears appropriately antialiased * [x] Validate the "\e[30;47m\u{2500} What \u{2500}\e[m" still works when `FillGeometry` is nerfed as a quick test that the runs are split correctly. * [x] Put `u{fffd} into Powershell Core to make a replacement char in the output. Then press enter a few times and see that shrunken initial characters on random rows. Verify this is gone. Closes #6668 Closes #6669 Co-Authored-By: Chester Liu <skyline75489@outlook.com>
claunia added the pull-request label 2026-01-31 09:18:04 +00:00
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