Issue 882283  has been merged into this issue.

+futhark: I took a deeper look here but I'm not sure what the right way to fix this is.

When an element is or is inside a position: fixed tree, we want 100vh to resize along with the URL bar. This should match %-based heights which get their size from the static (w.r.t. the url bar) ICB if they're non-fixed, and the actual viewport size (i.e. the FrameView's frame rect) if they are fixed.

It looks like we get and store the viewport size for vh units during style recalc. At this point, we don't yet know we're in a fixed-position element though, let alone a subtree. Is there a good way to do this from style or should we apply a special case in layout for this?

(for context, see https://developers.google.com/web/updates/2016/12/url-bar-resizing)

Actually, a special case in layout sounds bad since vh can be used in all sorts of places. Can we make it so that viewport size resolution happens after we've determined whether we're in a fixed subtree?

You can figure out if you're in a fixed subtree if you walk up ancestor computed styles, I guess. Inheriting a custom flag down computed styles in not ideal as that will cause subtree recalcs where we currently don't have (position changing). It's possible to lazily figure out if we are in a fixed subtree when we try to resolve viewport units by walking the flat tree ancestors' computed style for position:fixed.

I think that would mean keeping track of this down/up RecalcStyle() somehow, without needing to walk the ancestor chain every time. Similarly to pushing selector filters.

Do we already have the functionality in place for marking style dirty when the URL bar moves?

It's possible to just read the position() value out of ComputedStyle as you walk down the tree, but I suspect you'll see that affect performance doing small changes deep down an otherwise clean tree.

What if vw functionality was reverted to pre version 56 and a new "display height" was added that acts as vh currently does.  That way vh would work in a way that makes sense and there would be a unit that stays the same regardless of whether or not the menu bar is shown.