macOS: Highlight selected part of preedit using platform theme palette

QInputMethodEvent::Selection unfortunately doesn't apply to the
preedit text, and QInputMethodEvent::Cursor which does, doesn't
support setting a selection. Until we've introduced attributes
that allow us to propagate the preedit selection semantically
we resort to styling the selection via the TextFormat attribute,
so that the preedit selection is visible to the user.

This allows us to remove the fallback we had for thick and double
underline styles, where we mapped those to the wiggly underline style.
This was needed to distinguish the selected cluster when composing
CJK, but looked out of place.

One disadvantage of faking the selection via text format is that
we will not update the selection color on theme change, e.g. when
switching from light to dark mode, but this is a minor issue that we
can live with until we've introduced a proper QInputMethodEvent
attribute for the preedit selection.

Pick-to: 6.2
Change-Id: I1c45c310107697962e328a4db908d29d2358f756
Reviewed-by: Volker Hilsheimer <[email protected]>

src/plugins/platforms/cocoa/qnsview_complextext.mm

@@ -133,6 +133,22 @@ - (void)setMarkedText:(id)text selectedRange:(NSRange)selectedRange replacementR
     preeditAttributes << QInputMethodEvent::Attribute(
         QInputMethodEvent::Cursor, selectedRange.location + selectedRange.length, true);
 
+
+    // QInputMethodEvent::Selection unfortunately doesn't apply to the
+    // preedit text, and QInputMethodEvent::Cursor which does, doesn't
+    // support setting a selection. Until we've introduced attributes
+    // that allow us to propagate the preedit selection semantically
+    // we resort to styling the selection via the TextFormat attribute,
+    // so that the preedit selection is visible to the user.
+    QTextCharFormat selectionFormat;
+    auto *platformTheme = QGuiApplicationPrivate::platformTheme();
+    auto *systemPalette = platformTheme->palette();
+    selectionFormat.setBackground(systemPalette->color(QPalette::Highlight));
+    preeditAttributes << QInputMethodEvent::Attribute(
+        QInputMethodEvent::TextFormat,
+        selectedRange.location, selectedRange.length,
+        selectionFormat);
+
     int index = 0;
     int composingLength = preeditString.length();
     while (index < composingLength) {
@@ -166,11 +182,7 @@ - (void)setMarkedText:(id)text selectedRange:(NSRange)selectedRange replacementR
 
             // Unfortunately QTextCharFormat::UnderlineStyle does not distinguish
             // between NSUnderlineStyle{Single,Thick,Double}, which is used by CJK
-            // input methods to highlight the selected clause segments, so we fake
-            // it using QTextCharFormat::WaveUnderline.
-            if ((style & NSUnderlineStyleThick) == NSUnderlineStyleThick
-                || (style & NSUnderlineStyleDouble) == NSUnderlineStyleDouble)
-                format.setUnderlineStyle(QTextCharFormat::WaveUnderline);
+            // input methods to highlight the selected clause segments.
         }
         if (NSColor *underlineColor = attributes[NSUnderlineColorAttributeName])
             format.setUnderlineColor(qt_mac_toQColor(underlineColor));