Changing such tests to match the new intended behavior seems fine. Do we have use counters for how often the change would cause fullscreen to no longer work in the wild, though?

The old intended behavior should have blocked this though, right? There isn't any situation where activating only the main frame would allow a subframe to go fullscreen? (At least without some kind of gesture delegation?)

I suspect this is a bug fix, but if you think we need a counter for the behavior change anyway, I'm sure we could add one.

We have 3 failing tests in fullscreen/, for the exact same reason.

Here are 6 (=2*3) questions to foolip@ to understand the expectations, the answers are not obvious to me from the spec, may be because I am missing some implicit details around "browsing context".

Suppose we have frame P containing a child frame C, and P has just seen a user activation.  When they have
  (A) the same origin, and
  (B) different origins,
can:
  (1) P make an element in C fullscreen?
  (2) C make an element in C fullscreen?
  (3) C make an element in P fullscreen?

https://fullscreen.spec.whatwg.org/#allowed-to-request-fullscreen depends on https://html.spec.whatwg.org/multipage/interaction.html#triggered-by-user-activation which is known to be a bit wrong, so what the answers that come from reading that can't be trusted entirely.

But, it defines things in terms of tasks, and "The task in which the algorithm is running was queued" with a time limit, so I'd say that any short chain of setTimeout or postMessage same- or cross-origin should work per the current spec.

So, "P make an element in C fullscreen" by reaching into C directly or via postMessage, but C can only do things in a task that connects back to the original task in P. In other words, a setTimeout loop in C shouldn't succeed around the time of the click on P.

To make the (A) vs. (B) answer explicit, I can spot no explicit distinguish made between them as HTML is currently written.

As for what would make sense ignoring the spec, I think that all patterns that work for cross-origin should also work for same-origin, and that any same-origin patterns that don't work for cross-origin don't work because of some other existing limitation, not because of explicit cross-origin logic for user activation.

Such a limitation would be the inability to access the iframe.contentDocument for a cross-origin iframe, making it impossible to call requestFullscreen() inside that iframe synchronously from a click event in the parent frame.

Thanks, I will fix the tests according to #c4 (i.e. as per current activation spec).

Re #c5: This makes sense, thanks.  I still have one follow-up question from the perspective of our new frame-based user activation model (so please ignore current activation spec):

---

In the same origin case, if the parent frame's click handler calls
  iframeElem.contentDocument.documentElement.webkitRequestFullScreen(),
should the call succeed?

I thought the answer should be "yes" but then the "context" in the current FullScreen spec should include an "execution context" (who is making the call) in addition to the "context object" we have today.
https://fullscreen.spec.whatwg.org/#dom-element-requestfullscreen
Wdyt?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fdd52df96081dcb00b3fdf6ccb6726f60b49173c

commit fdd52df96081dcb00b3fdf6ccb6726f60b49173c
Author: Mustaq Ahmed <mustaq@google.com>
Date: Thu Nov 23 15:43:24 2017

Fix fullscreen layout-test scripts to match manual tests.

The manual parts of these tests ask for mouse clicks in subframes.  But
the scripts used to send key events to main frame instead.

Bug:  784999 
Change-Id: I84e72334c78cc6af981c4fb6d51a32bba6554a4c
Reviewed-on: https://chromium-review.googlesource.com/786298
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Mustaq Ahmed <mustaq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518946}
[modify] https://crrev.com/fdd52df96081dcb00b3fdf6ccb6726f60b49173c/third_party/WebKit/LayoutTests/fullscreen/exit-full-screen-iframe.html
[modify] https://crrev.com/fdd52df96081dcb00b3fdf6ccb6726f60b49173c/third_party/WebKit/LayoutTests/fullscreen/full-screen-iframe-allowed.html
[modify] https://crrev.com/fdd52df96081dcb00b3fdf6ccb6726f60b49173c/third_party/WebKit/LayoutTests/fullscreen/full-screen-iframe-not-allowed.html
[modify] https://crrev.com/fdd52df96081dcb00b3fdf6ccb6726f60b49173c/third_party/WebKit/LayoutTests/fullscreen/full-screen-iframe-ua-style.html
[add] https://crrev.com/fdd52df96081dcb00b3fdf6ccb6726f60b49173c/third_party/WebKit/LayoutTests/fullscreen/resources/inner-video.html
[modify] https://crrev.com/fdd52df96081dcb00b3fdf6ccb6726f60b49173c/third_party/WebKit/LayoutTests/fullscreen/resources/inner.html

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c32e5f1c178f70f950ab26ff5c08bad4c5c778f7

commit c32e5f1c178f70f950ab26ff5c08bad4c5c778f7
Author: Mustaq Ahmed <mustaq@google.com>
Date: Fri Nov 24 17:08:06 2017

Fixed click coordinates in one layout test.

Bug:  784999 
Change-Id: Ief9fe76301c214721584c8e51bd9f2225f7e0a3c
Reviewed-on: https://chromium-review.googlesource.com/788131
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Mustaq Ahmed <mustaq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519137}
[modify] https://crrev.com/c32e5f1c178f70f950ab26ff5c08bad4c5c778f7/third_party/WebKit/LayoutTests/fullscreen/model/fully-exit-fullscreen-nested-iframe.html
[modify] https://crrev.com/c32e5f1c178f70f950ab26ff5c08bad4c5c778f7/third_party/WebKit/LayoutTests/fullscreen/trusted-click.js

The only test that remain is fullscreen/api/element-request-fullscreen-two-iframes.html, see  Issue 627792 .  We won't fix this test until we have the new activation model in a shippable state.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/69a828594a7b4e83a35eaf0b4be417434880b588

commit 69a828594a7b4e83a35eaf0b4be417434880b588
Author: Mustaq Ahmed <mustaq@google.com>
Date: Mon Jan 15 15:50:10 2018

Fixed the click coordinate in a fullscreen layouttest.

Bug:  784999 ,  627792 
Change-Id: I120d97f3d9b248bfe3cb22f1ff89b43101fc17b4
Reviewed-on: https://chromium-review.googlesource.com/861748
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Mustaq Ahmed <mustaq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#529288}
[modify] https://crrev.com/69a828594a7b4e83a35eaf0b4be417434880b588/third_party/WebKit/LayoutTests/fullscreen/api/element-request-fullscreen-two-iframes.html