I am pretty sure it's not possible. I've also wanted to do something like you but there just seems to be no solution so I'd suggest you use different windows for each ui element and make those windows borderless so it wouldn't be drag-able and wouldn't clutter up the screen. You could also use that transparent bg feature for elements that are close to each other so you wouldn't have to create hundreds of windows for little buttons. I might be a little late for you but maybe some other poor soul could get some use out of this.