[css-overflow] Viewport's overflow is not defined when the root has display: none #12649
Description
https://drafts.csswg.org/css-overflow-3/#overflow-propagation
UAs must apply the
overflow-*values set on the root element to the viewport when the root element’sdisplayvalue is notnone. However, when the root element is an [HTML]<html>element (including XML syntax for HTML) whoseoverflowvalue isvisible(in both axes), and that element has as a child a<body>element whosedisplayvalue is also notnone, user agents must instead apply theoverflow-*values of the first such child element to the viewport.
The spec is clear that we don't propagate from the root element in that case.
The spec could be interpreted as still allowing to propagate from the <body> (being in a display: none subtree doesn't mean that it has display: none itself), but no browser does that, as seen here:
<!DOCTYPE html>
<html style="display: none">
<style>
::-webkit-scrollbar { background-color: #eff0f1; width: 12px; height: 12px; }
::-webkit-scrollbar-thumb { background: #898c8d }
</style>
<body style="overflow: scroll">Regardless, if both the root and the body have display: none, the spec is clear that we don't propagate from either of them.
So what overflow does the viewport use?
The obvious choice is the initial overflow: visible, which will behave as auto:
If visible is applied to the viewport, it must be interpreted as auto.
Since there is no box, it's hard to tell if browsers are using visible/auto or clip/hidden. Nobody uses scroll, though.