HelpText: Box size/placement beyond viewport limits

### Description of the bug

A user reported via CRM/Dynamics that the helptext may stretch beyond the limits of the viewport in some cases. From the images it seems to happen when scroll position places the button in about the middle of the viewport.

### Steps To Reproduce

1. Add lots of helptext
2. Open app on mobile (i.e. Safari, Chrome, Firefox, built-in browser from phone vendor)
3. Scroll, click on the helptext, observe the problem

### Additional Information

- https://digdir.slack.com/archives/C075WJSDVQC/p1781081397914959

Start by reproducing the issue on a mobile browser with many helptext elements, scrolling until the trigger is near the middle of the viewport. Then locate the HelpText component and its positioning and sizing logic; done means the helptext remains fully within the viewport after opening at different scroll positions.

Written by the indexing model from the issue text.