SUMMARY In Plasma, whilst holding super, you can right click drag on a window to resize according to the region of the window you select in, either vertically, horizontally or diagonally. I'd like to suggest this is done for selections in Spectacle with the select tool. STEPS TO REPRODUCE OBSERVED RESULT EXPECTED RESULT SOFTWARE/OS VERSIONS N/A
Unfortunately, this is not compatible with the existing ways to interact with the selection and I have not seen any complaints about the existing behavior. You can currently right click to reset the selection or right click and drag to reset and create a new selection.