Steps to reproduce:

1. Make sure that Vector (2022) skin is selected. For new accounts it's a default skin.
2. Go to Special:Homepage - if the screen size width <1500px, then the page will have three-column layout display. In the screenshot below the "Recent activity" graph overlaps the Mentorship module a little bit.

3. Change the screen width to 1500px - the layout changes to two-column format:

Note: Vector (Legacy) doesn't behave this way - resizing a screen will result in adjusting the width of the page elements and only at the width ~1025ox, the elements will be placed vertically.