Returning to this: >>54659
If the bubble metaphor was real - and since weapons generally have the same range in all directions, why their protection wasn't comparable to a bubble or a dome - then centers of bubbles could be located: it's the radius' starting point.
Just bomb the vicinity until the bubble is gone.
Bu defense isn't concentrated in that way, and it's not a hermetic seal.
So again, while the frontline is the result of the interaction of both forces, the primary reason why the front looks like that is the Russian movements. They have the initiative so the impact of their decisions is the more defining.