This appears in revision 1019, and I have not had an executable to try it out on yet.
From Stefan Löffler's comments on http://code.google.com/p/texworks/source/detail?r=1019
... Works like TW.target.selectionStart.
If there is no selection, returns the same
value as TW.taget.selectionStart.
Otherwise, typically returns either
TW.target.selectionStart or (TW.target.selectionStart + TW.target.selectionEnd),
depending on where the cursor is positioned.
TW.target.selectionStart + TW.target.selectionLength is probably meant.
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