I think I've found a bug in TW 5.1.10 (prerelease):
1) Go to
http://tiddlywiki.com/prerelease/#SelectWidget2) scroll down to the example for "multiple" (at the end of the tiddler)
3) clicking on an item in the listbox results in:
Uncaught TypeError: Cannot read property 'indexOf' of null
Platform: Win 10 Home, Chrome 47.0.2526.106 m
Also, a related usability question: when using "multiple", the $select widget produces a listbox instead of a dropdown. This is necessary, since dropdowns don't support multiple selection. However, there doesn't seem to be a way to specify the number of items to display in the listbox (the size="nnn" attribute). At least, it's not in the documentation.
-e