Here we have an IFrame in a container 'strip'. The IFrame is styled to have a width and height of 100% of the container.

When the page loads, JavaScript expands the container width to 100vw. The issue occurs whens clicking a link inside the IFrame. This causes the parent container to unexpectedly revert to the original width, 'resetting' the width property that had been applied with JavaScript.

The parent container has a CSS transition on the width property so that the initial change of width to 100vw is animated. Crucially, removing this CSS transition stops the container collapsing when clicking a link in the IFrame, suggesting that the issue is related to the transition property.