I have a scenario where a rectangle is drawn inside another rectangle.

Now the user is dragging, for example purposes, let's take the RIGHT border of the larger rectangle. I want to at the same time resize the inner rectangle as well, but it turns out it is not as trivial task as I thought so. In the image bellow is the expected result but if you look closely not only the rectangle expanded but it also moved for a certain distance to the right!

Now I tried and tried to find a formula how much to move the inner shape before expanding it to the right but continuously failed... Just scaling the inner rectangle is not a good solution because if there is another rectangle inside next to it, the first one will go over the second one if they do not move for a certain distance like bellow:

The question is valid only for one-sided drag and expand, not for all sides scale by keeping aspect ratio... Any input is appreciated.