`amp-story-desktop-one-panel` small white lines on the side of panels when at certain screen sizes

### Description

Small white lines are visible on the side of the story panel when at certain screen sizes.
This is most visible when scaling a window.
Most often this happens on the right and bottom edges.

### Reproduction Steps

1. Start the dev server.
2. Be sure the `amp-story-desktop-pne-panel` experiment is on.
3. Load the `/examples/amp-story/ampconf.html` and slowly scale browser
4. Slowly scale the screen and see the lines appear.

Related to #29583