Fix underline color in diff description when hovering

Since e.g. whitespace can have a different foreground color than
"normal" text, we have to explicitly set the color for the underline
when hovering. Use foreground color if there is no specific text color
set for "normal" text.

Change-Id: I9825d24af0d598b039a0db9ed86966605e22ac04
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

src/plugins/diffeditor/diffeditor.cpp

@@ -34,6 +34,7 @@
 #include <coreplugin/minisplitter.h>
 
 #include <texteditor/displaysettings.h>
+#include <texteditor/fontsettings.h>
 #include <texteditor/marginsettings.h>
 #include <texteditor/syntaxhighlighter.h>
 #include <texteditor/textdocument.h>
@@ -198,7 +199,9 @@ void DescriptionEditorWidget::highlightCurrentContents()
     QTextEdit::ExtraSelection sel;
     sel.cursor = m_currentCursor;
     sel.cursor.select(QTextCursor::LineUnderCursor);
-    sel.format.setFontUnderline(true);
+    sel.format.setUnderlineStyle(QTextCharFormat::SingleUnderline);
+    const QColor textColor = TextEditorSettings::fontSettings().formatFor(C_TEXT).foreground();
+    sel.format.setUnderlineColor(textColor.isValid() ? textColor : palette().color(QPalette::Foreground));
     setExtraSelections(TextEditorWidget::OtherSelection,
                        QList<QTextEdit::ExtraSelection>() << sel);
 }