[whatwg] Page reflow when entering/exiting fullscreen

[Matt Falkenhagen]

When an element goes fullscreen, should the browser try to retain the
layout of the underlying document, e.g., by creating a placeholder for
the element?

With the top layer, it's possible to style the fullscreened element and
::backdrop such that the underlying document is visible. In such a case,
it may be surprising for the user to see the layout change. The scroll
position may also change.

As far as I can tell, the Fullscreen spec doesn't say anything normative
about retaining the layout. I'm wondering whether it was a deliberate
decision to leave it up to the author to create a placeholder if
desired. (As an implementer, this would be easier for me.)

WebKit and Blink actually already use a placeholder to prevent the page
from reflowing when entering/exiting fullscreen, but there are some
limitations. For example, inline elements don't get placeholders.