In Paragraph::changeCase, correctly track the font of the changed characters (fix crash)

Fixes: #7943

src/Paragraph.cpp

@@ -3304,7 +3304,9 @@ void Paragraph::changeCase(BufferParams const & bparams, pos_type pos,
 	// process sequences of modified characters; in change
 	// tracking mode, this approach results in much better
 	// usability than changing case on a char-by-char basis
-	docstring changes;
+	// We also need to track the current font, since font
+	// changes within sequences can occur.
+	vector<pair<char_type, Font> > changes;
 
 	bool const trackChanges = bparams.trackChanges;
 
@@ -3338,7 +3340,7 @@ void Paragraph::changeCase(BufferParams const & bparams, pos_type pos,
 		}
 
 		if (oldChar != newChar) {
-			changes += newChar;
+			changes.push_back(make_pair(newChar, getFontSettings(bparams, pos)));
 			if (pos != right - 1)
 				continue;
 			// step behind the changing area
@@ -3347,9 +3349,8 @@ void Paragraph::changeCase(BufferParams const & bparams, pos_type pos,
 
 		int erasePos = pos - changes.size();
 		for (size_t i = 0; i < changes.size(); i++) {
-			insertChar(pos, changes[i],
-				   getFontSettings(bparams,
-						   erasePos),
+			insertChar(pos, changes[i].first,
+				   changes[i].second,
 				   trackChanges);
 			if (!eraseChar(erasePos, trackChanges)) {
 				++erasePos;

status.20x

@@ -96,6 +96,9 @@ What's new
 
 - Fix crash when CheckTeX hits mathed (bug 8798).
 
+- Fix crash when performing "Change Case" operation on a selection with font
+  changes (bug 7943).
+
 - Fix hang when selecting text (bug 8837).
 
 - Fix assertion when entering a path into the import dialog (bug 7437).