Windowsize enlarge dynamicly

I want to make I Rule like the Tool SHIFTIT, there You have the option to Add px to the Window.
In BBT you can also do that, but only at the right side. If the Window is on the right frame of the window but left is space the enlarg nothing is done.
Please add the option to also enlarge at the left side automaticly if the right ist "full"

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