This prevents stale drawing from being revealed if the window later grows.

For instance, 19 ppem "VL Gothic" on Windows, the advance value is 10
for half-width characters, but 18 for full-width characters.  In this
case, tmAveCharWidth is 9. Thus using tmAveCharWidth is reasonable.

This lets programs ask the user to save documents, etc., and possibly cancel
the quit.

Some events are application-wide, not specific to a thread.  Such an event
needs to be broadcast to all GUI-attached threads because we don't know which
are handling events, but we don't want the event to be processed in each.
Often it should only be processed by the first to pull it from its queue.