Make sure num_glyphs pass to HarfBuzz is large enough

(Backport from master)

Currently we only pass the num_glyphs for the run to
HB_ShapeItem, but it can be less then the string length
for this run because of Unicode surrogates. Thus, we need
to pass at least the length of that run as num_glyphs to
HB (given that we have enough space allocated because for
the entire string), if that's still not enough, we will
do ensureSpace again according to the num_glyphs returned
by HB and move remaining glyphs backwards.

Task-number: QTBUG-15679
Reviewed-by: Lars Knoll

1 files changed, 4 insertions, 1 deletions

diff --git a/src/gui/text/qtextengine.cpp b/src/gui/text/qtextengine.cpp
index b511b5a..4378c62 100644
--- a/src/gui/text/qtextengine.cpp
+++ b/src/gui/text/qtextengine.cpp
@@ -1233,6 +1233,8 @@ void QTextEngine::shapeTextWithHarfbuzz(int item) const
             shaper_item.num_glyphs -= itemBoundaries[k + 1];
         }
         shaper_item.initialGlyphCount = shaper_item.num_glyphs;
+        if (shaper_item.num_glyphs < shaper_item.item.length)
+            shaper_item.num_glyphs = shaper_item.item.length;
 
         QFontEngine *actualFontEngine = font;
         uint engineIdx = 0;
@@ -1257,7 +1259,8 @@ void QTextEngine::shapeTextWithHarfbuzz(int item) const
             }
 
             const QGlyphLayout g = availableGlyphs(&si).mid(glyph_pos);
-            moveGlyphData(g.mid(shaper_item.num_glyphs), g.mid(shaper_item.initialGlyphCount), remaining_glyphs);
+            if (shaper_item.num_glyphs > shaper_item.item.length)
+                moveGlyphData(g.mid(shaper_item.num_glyphs), g.mid(shaper_item.initialGlyphCount), remaining_glyphs);
 
             shaper_item.glyphs = g.glyphs;
             shaper_item.attributes = g.attributes;