See if this can kickstart something. It’s a crude implementation and the numbers would need tweaking but it could be a start:
The widget is just a 32x32 border drawn as Box with big margins:
And it clamp-scales based on the distance to the target and its size:
vid: https://i.gyazo.com/ab795c549cd50eb7…497f31a53e.mp4
Again, the numbers need tweaking. May need extra work if you need the bracket non square.