Use QTextCharFormat::fontFixedPitch to remember Markdown backtick spans

If the editing app (like qtbase/examples/widgets/richtext/textedit)
has controls only for setting a specific font, and someone uses it to
write markdown "from scratch", then we need to detect that they chose
Courier or some other fixed-pitch font, and write the backticks,
because Markdown has no syntax for selecting a specific font family.
If the user loads markdown into such an application, the font
is set to QFontDatabase::systemFont(QFontDatabase::FixedFont).
Round-trip editing was already working, as long as such a font exists.
QTextCharFormat::setFont() calls setFontFixedPitch(font.fixedPitch()),
but for the chosen "mono" font, font.fixedPitch() can be false.
For semantic completeness and separation of concerns, we now
set fontFixedPitch explicitly if a `backtick` span is encountered.
As a followup to f1e60de665 this
should get its autotest passing reliably.

Fixes: QTBUG-99676
Pick-to: 6.3 6.2 5.15
Change-Id: I4987a1f0f819f82ec64546bdc3ef53e7d29933de
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

src/gui/text/qtextmarkdownimporter.cpp

@@ -426,6 +426,7 @@ int QTextMarkdownImporter::cbEnterSpan(int spanType, void *det)
     }
     case MD_SPAN_CODE:
         charFmt.setFont(m_monoFont);
+        charFmt.setFontFixedPitch(true);
         break;
     case MD_SPAN_DEL:
         charFmt.setFontStrikeOut(true);

src/gui/text/qtextmarkdownwriter.cpp

@@ -532,7 +532,7 @@ int QTextMarkdownWriter::writeBlock(const QTextBlock &block, bool wrap, bool ign
             col += s.length();
         } else {
             QFontInfo fontInfo(fmt.font());
-            bool monoFrag = fontInfo.fixedPitch();
+            bool monoFrag = fontInfo.fixedPitch() || fmt.fontFixedPitch();
             QString markers;
             if (!ignoreFormat) {
                 if (monoFrag != mono && !m_indentedCodeBlock && !m_fencedCodeBlock) {

tests/auto/gui/text/qtextmarkdownimporter/tst_qtextmarkdownimporter.cpp

@@ -77,6 +77,7 @@ public:
         Mono = 0x10,
         Link = 0x20
     };
+    Q_ENUM(CharFormat)
     Q_DECLARE_FLAGS(CharFormats, CharFormat)
 };
 
@@ -364,13 +365,15 @@ void tst_QTextMarkdownImporter::nestedSpans()
                          << "underlined" << fmt.fontUnderline()
                          << "strikeout" << fmt.fontStrikeOut() << "anchor" << fmt.isAnchor()
                          << "monospace" << QFontInfo(fmt.font()).fixedPitch() // depends on installed fonts (QTBUG-75649)
-                                        << fmt.fontFixedPitch() // returns false even when font family is "monospace"
-                                        << fmt.hasProperty(QTextFormat::FontFixedPitch); // works
+                                        << fmt.fontFixedPitch()
+                                        << fmt.hasProperty(QTextFormat::FontFixedPitch)
+                         << "expected" << expectedFormat;
         QCOMPARE(fmt.fontWeight() > QFont::Normal, expectedFormat.testFlag(Bold));
         QCOMPARE(fmt.fontItalic(), expectedFormat.testFlag(Italic));
         QCOMPARE(fmt.fontUnderline(), expectedFormat.testFlag(Underlined));
         QCOMPARE(fmt.fontStrikeOut(), expectedFormat.testFlag(Strikeout));
         QCOMPARE(fmt.isAnchor(), expectedFormat.testFlag(Link));
+        QCOMPARE(fmt.fontFixedPitch(), expectedFormat.testFlag(Mono));
         QCOMPARE(fmt.hasProperty(QTextFormat::FontFixedPitch), expectedFormat.testFlag(Mono));
         ++iterator;
     }