Try to match variant-selector font to preceding character

Variant-selectors are special unicode symbols which are used to
modify glyph selection for the preceding character. For instance,
a regular symbol could be turned into a color emoji using VS16,
the emoji variation selector. In order for this to work, however,
the font that handles the selector has to handle the full pair of
characters, so that it can apply the correct substitution rules.

One specific example of this was on macOS, where an airplane
symbol + VS16 would match the symbol to the default UI font but
the VS16 to the emoji font. Since there string provided for the
emoji font did not have any preceding character for VS16, we just
ignored it.

To improve on this, we now detect variation selectors that have
been matched to different font engines than the preceding
character. When such a case occurs, we check if the selector font
also supports the preceding character, and if it does, we keep
the pair together and use the same font for both.

[ChangeLog][QtGui][Text] Fix some cases where a variation-selector
character would be ignored in font selection and the correct
variant of a character would thus not be selected.

Task-number: QTBUG-108799
Change-Id: I9f427e0520e652ee2f24a4f7dc3c1957251e06bd
Reviewed-by: Tor Arne Vestbø <[email protected]>

3 files changed, 32 insertions, 2 deletions

diff --git a/src/gui/text/qfontengine.cpp b/src/gui/text/qfontengine.cpp
index 61705842dbd..8b0dff31af7 100644
--- a/src/gui/text/qfontengine.cpp
+++ b/src/gui/text/qfontengine.cpp
@@ -1825,6 +1825,7 @@ bool QFontEngineMulti::stringToCMap(const QChar *str, int len,
     QStringIterator it(str, str + len);
 
     int lastFallback = -1;
+    char32_t previousUcs4 = 0;
     while (it.hasNext()) {
         const char32_t ucs4 = it.peekNext();
 
@@ -1886,10 +1887,35 @@ bool QFontEngineMulti::stringToCMap(const QChar *str, int len,
                     break;
                 }
             }
+
+            // For variant-selectors, they are modifiers to the previous character. If we
+            // end up with different font selections for the selector and the character it
+            // modifies, we try applying the selector font to the preceding character as well
+            const int variantSelectorBlock = 0xFE00;
+            if ((ucs4 & 0xFFF0) == variantSelectorBlock && glyph_pos > 0) {
+                int selectorFontEngine = glyphs->glyphs[glyph_pos] >> 24;
+                int precedingCharacterFontEngine = glyphs->glyphs[glyph_pos - 1] >> 24;
+
+                if (selectorFontEngine != precedingCharacterFontEngine) {
+                    QFontEngine *engine = m_engines.at(selectorFontEngine);
+                    glyph_t glyph = engine->glyphIndex(previousUcs4);
+                    if (glyph != 0) {
+                        glyphs->glyphs[glyph_pos - 1] = glyph;
+                        if (!(flags & GlyphIndicesOnly)) {
+                            QGlyphLayout g = glyphs->mid(glyph_pos - 1, 1);
+                            engine->recalcAdvances(&g, flags);
+                        }
+
+                        // set the high byte to indicate which engine the glyph came from
+                        glyphs->glyphs[glyph_pos - 1] |= (selectorFontEngine << 24);
+                    }
+                }
+            }
         }
 
         it.advance();
         ++glyph_pos;
+        previousUcs4 = ucs4;
     }
 
     *nglyphs = glyph_pos;
diff --git a/tests/auto/gui/text/qglyphrun/tst_qglyphrun.cpp b/tests/auto/gui/text/qglyphrun/tst_qglyphrun.cpp
index b0d6efc065e..efd590a730a 100644
--- a/tests/auto/gui/text/qglyphrun/tst_qglyphrun.cpp
+++ b/tests/auto/gui/text/qglyphrun/tst_qglyphrun.cpp
@@ -633,8 +633,12 @@ void tst_QGlyphRun::defaultIgnorables()
 
         bool hasFullMainFontRun = false;
         for (const QGlyphRun &run : runs) {
+            // QtsSpecialFont will be used for at least five characters: AA[...]111
+            // Depending on the font selected for the 0xFE0F variant selector, the
+            // third 'A' may be in QtsSpecialFont or in the fallback. This is platform-specific,
+            // so we accept either.
             if (run.rawFont().familyName() == QStringLiteral("QtsSpecialTestFont")
-                    && run.glyphIndexes().size() == 6) {
+                    && run.glyphIndexes().size() >= 5) {
                 hasFullMainFontRun = true;
                 break;
             }
diff --git a/tests/baseline/painting/scripts/text.qps b/tests/baseline/painting/scripts/text.qps
index 4d81b3084c7..6bacdfd5e62 100644
--- a/tests/baseline/painting/scripts/text.qps
+++ b/tests/baseline/painting/scripts/text.qps
@@ -165,7 +165,7 @@ translate 0 75
 save
   setPen black
   setFont "sansserif" 16 normal
-  drawText 0 40 "e😃m😇o😍j😜i😸!"
+  drawText 0 40 "e😃m😇o😍j😜i😸!✈️"
 restore
 
 translate 0 75