Fix Korean IME overlay hiding characters to the right of cursor#20041
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DHowett merged 5 commits intomicrosoft:mainfrom Apr 17, 2026
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…cursor When composing a new character between two existing characters, the in-progress composition text was drawn over the character immediately to the right of the cursor, visually hiding it during composition. This happened because the tsfPreview overlay used an overwrite-style render, writing the composition text at the cursor column without preserving the displaced content. The fix copies the original characters from that position back into the visual row at the position just after the composition text ends, changing the appearance from overwrite to insert. The buffer itself is not modified; the change is local to the temporary row modification that is restored after each paint frame. This regression was introduced by commit a3d508a (Remove TF_TMAE_UIELEMENTENABLEDONLY, microsoft#19738), which caused the Korean IME to switch from IMM32 (which renders its own floating composition window) to TSF (which uses the terminal's inline tsfPreview rendering), exposing this rendering flaw.
Reverts the change that shifted existing characters to the right of the cursor when rendering an in-progress TSF composition (tsfPreview). The approach was an unconditional rightward shift of all content from the cursor position, which caused box-drawing characters in TUI applications to be displaced and rendered at wrong positions or cut off entirely. Example of the breakage in a TUI text box: Before (correct): |Hello [ㄷ] | After (broken): |Hello [ㄷ]Worl|d (border shifted off screen) The correct fix requires consuming an equal amount of whitespace to the right of the composition before shifting, so that box borders remain at their original column positions when trailing whitespace is available. This improved approach will be implemented separately as Fix 2-1. Reviewer feedback: When you're in a TUI application, there may be a box around the text field. Shifting text from the cursor to the right means that the right border (and anything else in the TUI app) also shifts to the right. One solution for this is that we try to remove an equal amount of whitespace to the right of the composition.
…cursor When composing a new Korean character between existing text, the in-progress composition overlay was drawn using an overwrite-style render that hid any character immediately to the right of the cursor. This is visually incorrect for Korean IME, which inserts rather than replaces. The fix copies original characters from the cursor position back to the right of the composition text, using a whitespace-absorption algorithm so that TUI box borders stay at their original column positions when trailing whitespace is available. Up to W whitespace columns (where W is the composition width) are consumed silently; remaining non-whitespace content is shifted rightward. If no whitespace is available, the shift is unavoidable (border may move), but this is the best achievable behavior for fixed-width lines. The rendering change is purely visual: the buffer row is restored in full after each paint frame by the existing wil::scope_exit restore mechanism. This approach was suggested by a reviewer as a refinement to a prior attempt that shifted all content unconditionally and broke TUI box borders. Closes microsoft#20040
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DHowett
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lhecker
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Mar 31, 2026
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The compositionRow code is now rather lengthy with this new code. I think it makes sense to hoist it into its own dedicated function if possible. But the changes overall look good to me. 🙂
Let me think about this over the next few days! I'll check this PR out locally.
lhecker
reviewed
Apr 16, 2026
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The block above is identical to before.
DHowett
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Apr 16, 2026
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April 16, 2026 23:43
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This commit fixes a regression in v1.24, where composing Korean text
in-between existing characters causes text to the right to be hidden.
This is problematic for the Korean IME, because it commonly inserts
the composed syllable between existing characters.
The fix compresses (removes/absorbs) available whitespace to the right
of the composition to make space for any existing text. This means
that a composition now virtually acts in insert mode.
Closes #20040
Refs #19738
Refs #20039
Validation Steps Performed
ㄷbetween가and나→ display shows가ㄷ나,나visible.Co-authored-by: Leonard Hecker lhecker@microsoft.com