This is a modal dialog with backdrop.

The `scrollbar-gutter: stable` causes the dialog mask to not cover the viewport, showing a red background on the right side.

Normally when opening a modal dialog we want the page behind it not to be scrollable, which requires adding `html { overflow: hidden; }`, but since the scrollbar is hidden it causes the content of the page to be offset to the right.

`scrollbar-gutter: stable` solves this problem, but after opening the dialog you will notice a red background on the right side, which is very strange.