Fix #20628: Stop caret left moving off the input string

distribution/changelog.txt

@@ -4,6 +4,7 @@
 - Improved: [#20951] Activate OpenRCT2 window after using native file dialog on macOS.
 - Fix: [#20255] Images from the last hovered-over coaster in the object selection are not freed.
 - Fix: [#20616] Confirmation button in the track designer’s quit prompt has the wrong text.
+- Fix: [#20628] Moving caret using Ctrl+left can move too far when using a multibyte grapheme.
 - Fix: [#21145] [Plugin] setInterval/setTimeout handle conflict.
 - Fix: [#21157] [Plugin] Widgets do not redraw correctly when updating disabled or visibility state. 
 - Fix: [#21158] [Plugin] Potential crash using setInterval/setTimeout within the callback.

src/openrct2-ui/TextComposition.cpp

@@ -276,7 +276,8 @@ void TextComposition::CaretMoveToLeftToken()
             lastChar = selectionOffset;
             break;
         }
-
+        if (selectionOffset == 0)
+            break;
         ch--;
         selectionOffset--;
     }
@@ -295,12 +296,13 @@ void TextComposition::CaretMoveToLeftToken()
             break;
 
         lastChar = selectionOffset;
-
+        if (selectionOffset == 0)
+            break;
         ch--;
         selectionOffset--;
     }
 
-    _session.SelectionSize = std::max<size_t>(0, _session.SelectionSize - (selectionOffset - _session.SelectionStart));
+    _session.SelectionSize = _session.SelectionSize - (selectionOffset - _session.SelectionStart);
     _session.SelectionStart = selectionOffset == 0 ? 0 : lastChar;
 }