Don't do font merging for PUA characters

The "Private Use Area" are subsets of Unicode which are not
considered regular characters, but reserved for fonts to provide
custom glyphs.

If these were used and the main font did not have support for them,
we would look them up in other fonts and sometimes display an
arbitrary selection of glyphs, based on whatever existed on the
platform. This is unexpected and different from how native apps
work on Windows, for instance.

[ChangeLog][QtGui][Text] Font merging (automatic assignment of
alternative fonts) is no longer applied for characters in the
Private Use Areas of Unicode.

Fixes: QTBUG-110502
Change-Id: Id2c63786aafda59bf170e0d7263eb78a391fe46d
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit fc33fea999365c36ed446eee0db0d59d94be306b)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

2 files changed, 41 insertions, 1 deletions

diff --git a/src/gui/text/qfontengine.cpp b/src/gui/text/qfontengine.cpp
index 6b764dee54..5035b61fe9 100644
--- a/src/gui/text/qfontengine.cpp
+++ b/src/gui/text/qfontengine.cpp
@@ -1862,7 +1862,8 @@ bool QFontEngineMulti::stringToCMap(const QChar *str, int len,
                 && ucs4 != QChar::LineSeparator
                 && ucs4 != QChar::LineFeed
                 && ucs4 != QChar::CarriageReturn
-                && ucs4 != QChar::ParagraphSeparator) {
+                && ucs4 != QChar::ParagraphSeparator
+                && QChar::category(ucs4) != QChar::Other_PrivateUse) {
             if (!m_fallbackFamiliesQueried)
                 const_cast<QFontEngineMulti *>(this)->ensureFallbackFamiliesQueried();
             for (int x = 1, n = qMin(m_engines.size(), 256); x < n; ++x) {
diff --git a/tests/auto/gui/text/qtextscriptengine/tst_qtextscriptengine.cpp b/tests/auto/gui/text/qtextscriptengine/tst_qtextscriptengine.cpp
index f28478dafd..d3267d024b 100644
--- a/tests/auto/gui/text/qtextscriptengine/tst_qtextscriptengine.cpp
+++ b/tests/auto/gui/text/qtextscriptengine/tst_qtextscriptengine.cpp
@@ -56,6 +56,9 @@ private slots:
 
     void shapingDisabledDevanagari();
     void shapingDisabledLatin();
+
+    void privateUseArea();
+
 private:
     bool haveTestFonts;
 };
@@ -1312,5 +1315,41 @@ void tst_QTextScriptEngine::shapingDisabledDevanagari()
     QCOMPARE(noShapingRuns.first().glyphIndexes().size(), normalRuns.first().glyphIndexes().size());
 }
 
+void tst_QTextScriptEngine::privateUseArea()
+{
+    QString privateUseAreaString = QString::fromUtf8("");
+
+    QFont font;
+    QList<QGlyphRun> withFontMerging;
+    {
+        QTextLayout layout;
+        layout.setText(privateUseAreaString);
+        layout.setFont(font);
+        layout.beginLayout();
+        layout.createLine();
+        layout.endLayout();
+
+        withFontMerging = layout.glyphRuns();
+    }
+
+    font.setStyleStrategy(QFont::NoFontMerging);
+    QList<QGlyphRun> withoutFontMerging;
+    {
+        QTextLayout layout;
+        layout.setText(privateUseAreaString);
+        layout.setFont(font);
+        layout.beginLayout();
+        layout.createLine();
+        layout.endLayout();
+
+        withoutFontMerging = layout.glyphRuns();
+    }
+
+    QCOMPARE(withFontMerging.size(), withoutFontMerging.size());
+
+    for (int i = 0; i < withFontMerging.size(); ++i)
+        QCOMPARE(withFontMerging.at(i).glyphIndexes(), withoutFontMerging.at(i).glyphIndexes());
+}
+
 QTEST_MAIN(tst_QTextScriptEngine)
 #include "tst_qtextscriptengine.moc"