Last point first...
Rolling-back to an earlier version of iOS/iPadOS is not possible.
All System Updates are digitally signed by Apple - and can only be installed whilst the digital signatures remain valid. After release of a new iOS/iPadOS version, the immediately preceding version continues to be digitally signed by Apple - however, usually within a few days, Apple will revoke the digital certificate with which the earlier version is signed.
Perhaps consider that iOS/iPadOS updates include both functional and essential security updates. Restoring an earlier version of the OS would undermine the security of your device and your personal data.
Moving on to unexpected resizing of App Windows...
If you inadvertently reduce s full-screen App to a window, you will see a traffic-light button at top-left of the App Window. Tap this button to expand the control to three individual buttons:
- tap the green button to expand the window to full screen.
- tap the yellow button to minimise the App window, but leave the App running.
- tap the red button to close the App and its associated App window.
When scrolling a Safari window, avoid the extreme corners of the screen - as touching and dragging a corner with resize the window. If you do accidentally reduce the full-screen to a smaller window, just tap the traffic-light button as described above, then tap the green button to restore the screen. Alternatively, you can touch-and-hood the crescent "handle" at bottom-right of the App window - and drag to the bottom-right corner of the screen.
Remember, for iPadOS 26, you have a configuration choice to use Full Screen Apps only, use Windowed Apps, or Stage Manager mode as you prefer:
Settings > Multitasking & Gestures - select your preferred multitasking mode
Choosing Full-Screen Apps mode from iPad settings will fully disable multitasking:
