Ah, I see. Then yes, you should be able to predict the the desired size with force prepass and offset it accordingly.
Alternatively, you can read the location of the cursor and compare it to viewport size. If the location of the cursor + widget size would exceed the dimensions of the screen, offset the widget position by the desired distance from the screen edge.
The above is a decent method if you want the icons to stick to the edge of the screen - like indicators for objects that have moved off-screen. Or for keeping a widget inside the screen space even if the object has left it.
Anyhow, glad you found a solution.
Good luck with the rest!