Add magic comment. (#2010)

Related comment: https://github.com/Textualize/textual/pull/2003\#discussion_r1131081129

Related PR: #2003.

src/textual/driver.py

@@ -55,6 +55,7 @@ class Driver(ABC):
                 and not event.button
                 and self._last_move_event is not None
             ):
+                # Deduplicate self._down_buttons while preserving order.
                 buttons = list(dict.fromkeys(self._down_buttons).keys())
                 self._down_buttons.clear()
                 move_event = self._last_move_event