I think that with  issue 240576  fixed, it's possible that the scrolling will no longer be needed. After all, one can open a <dialog> element at any scroll offset and it will cover the whole screen.

However, the resize itself will change the layout, but I'm not sure how that interacts with scrolling, is there any code that tries to stay in the same position while resizing?

Although I am not confident that  issue 240576  will unblock this, I'm marking it as blocked, because I'm pretty sure that trying to fix this with the current code is not time well spent.

Scroll anchoring tries to anchor during resize, but ideally we would suspend layout of content outside the fullscreen element, while we're in fullscreen mode.

The fullscreen element itself need not be opaque, so the bits below it can be visible. That doesn't make much sense of course, and if there's a performance argument to be made one might consider changing the spec to allow some optimization or another.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0de9638b5d008eb11b206187bf90a70bbf23857a

commit 0de9638b5d008eb11b206187bf90a70bbf23857a
Author: Dave Tapuska <dtapuska@chromium.org>
Date: Wed Aug 29 15:08:54 2018

Avoid scrolling when entering fullscreen

When entering fullscreen we shouldn't need to set scroll to 0,0. This
fixes fullscreening an element that has scroll offset.

BUG= 693075 , 684920 

Change-Id: I2acf27eb2641731d580865d38e1a1495f6aee7de
Reviewed-on: https://chromium-review.googlesource.com/1191664
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587120}
[add] https://crrev.com/0de9638b5d008eb11b206187bf90a70bbf23857a/third_party/WebKit/LayoutTests/external/wpt/fullscreen/rendering/fullscreen-root-block-scroll-manual.html
[modify] https://crrev.com/0de9638b5d008eb11b206187bf90a70bbf23857a/third_party/blink/renderer/core/frame/fullscreen_controller.cc