First, uncheck "Settings->Environment->View->Enhanced multi-monitor dialog placement"
Then, edit the "default.conf" file in your settings directory.
Run a search for "Watches" and scroll (right) to the floatx and floatx attributes. Setting them both to 0 should work.
This is all just a guess, though, since I don't have multi-monitors.
[edit]
Actually, I don't know how the config setting works, so you may just be able to check/uncheck it to get the window back.
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