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Avoid splitting surrogate pairs when breaking runs for scaling #4731

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Feb 26, 2020
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Avoid splitting surrogate pairs when breaking runs for scaling #4731

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7 changes: 5 additions & 2 deletions src/renderer/dx/CustomTextLayout.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -361,13 +361,16 @@ CustomTextLayout::CustomTextLayout(gsl::not_null<IDWriteFactory1*> const factory
// If scale corrections were needed, we need to split the run.
for (auto [index, scale] : _glyphScaleCorrections)
{
// Don't split in the middle of a surrogate pair.
const auto after = IS_HIGH_SURROGATE(_text.at(index)) ? 2 : 1;

// Split after the adjustment first so it
// takes a copy of all the run properties before we modify them.
// GH 4665: This is the other half of the potential future perf item.
// If glyphs needing the same scale are coalesced, we could
// break fewer times and have fewer runs.
_SetCurrentRun(index + 1);
_SplitCurrentRun(index + 1);
_SetCurrentRun(index + after);
_SplitCurrentRun(index + after);

// Now split just this glyph off.
_SetCurrentRun(index);
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