Steps to reproduce:
1. Open my attached project "modalwindow".
2. Enter play mode in "Click Me" scene.
3. Press "Open modalwindow 1" button to open "Modal 1".
4. In game window press "Add more text" button.
--- Press it more than 10 times for scrollbar to appear.
--- Notice that scrollbar is located at the top.
5. Press "Open modalwindow 2" button to open "modal 2".
6. Press "Add more text" button more than 10 times.
7. Drag scrollbar to the bottom.
8. Press "x" button.
--- Notice that in opened "modal 1" window scrollbar is now located at the bottom.

Expected results: "Modal 1" windows scrollbar should be at the top.
Actual results: "Modal 1" windows scrollbar is at the bottom.

Reproduced with: 5.4.0f1, 5.4.0b25, 5.3.5p8, 5.3.5f1, 5.2.0f3