What you want seems to be a bug of that feature to me and I reported a similar thing in
Desktop Contact windows doesn't become opaque on mouse hover. You can make good use of that
bug until it is fixed (I hope it'll be fixed).
Also as I said in that thread already, when you hover your mouse on them, they are not fully opaque anyway. You can very hardly see behind of them. Maybe a registry setting for a 'transparency level when mouse over' can be implemented in the next versions. Patchou, the developer of Plus!, will most probably read this.
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