List of steps to reproduce (step by step, including full links if applicable):

• Find a small phone, or use responsive design mode to simulate it, set its width to 320 px (default width of PopupWidget)
• Open The official OOUI demo
• Click the "i" icon, then scroll up/down a bit

What happens?:
The content of the popup widget will move swiftly like a bird, leaving an empty box. This happens if screen width is close to or smaller than the popup box.

I believe this is a blocker of T268177.

What should have happened instead?:
The content can stay there and maybe display a horizontal scrollbar, instead of losing its composure and running away.

Software version (if not a Wikimedia wiki), browser information, screenshots, other information, etc.: tried both product and development version