Note: I added a TODO in fast/dom/Window/resources/window-property-collector.js when landing https://codereview.chromium.org/2025393003/ that we should fix when fixing this issue. The problem is that we always return the FrameHost's VisualViewport which means that there's no way to create a default "null" `visualViewport` for when the frame is navigated away or destroyed. Part of the work here should be to split the "DOMVisualViewport" from Blink's "VisualViewport" class.

I think this gets a little bit confusing since if the page is zoomed in (visual viewport != layout viewport) and the developer does something like iframe.contentWindow.visualViewport.pageScale, technically they should see '1', but what they want in the frame host's pageScale?

Are you suggesting the following values for iframe.contentWindow.visualViewport:
scrollTop: same as iframe.contentDocument.scrollingElement.scrollTop
scrollLeft: same as iframe.contentDocument.scrollingElement.scrollLeft
clientWidth: same as iframe.contentDocument.scrollingElement.clientWidth
clientHeight: same as iframe.contentDocument.scrollingElement.clientHeight
pageScale: 1

Also, setting pageScale does nothing here?

Actually, scrollTop and scrollLeft above should be 0 because pageScale is 1?

Right, scroll scrollTop and scrollLeft values should remain zero. The clientWidth/clientHeight should match iframe.contentDocument.scrollingElement.clientWidth/clientHeight.

Basically, we should treat each iframe as a separate viewport. If the user zooms into the page, they're scaling only the top-level viewport, so the child viewports shouldn't change at all.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/81dd83a5bae93da969e909c58ed3ecb50d50100c

commit 81dd83a5bae93da969e909c58ed3ecb50d50100c
Author: ymalik <ymalik@chromium.org>
Date: Thu Jun 23 15:59:05 2016

Seperate DOMVisualViewport from Blink's VisualViewport class

DOMVisualViewport represents the things exposed via the VisualViewportAPI. This
is what's returned from window.visualViewport. It queries the Blink's
VisualViewport for the main frame and returns sane values for iframes.

BUG= 604928 

Review-Url: https://codereview.chromium.org/2070563002
Cr-Commit-Position: refs/heads/master@{#401616}

[modify] https://crrev.com/81dd83a5bae93da969e909c58ed3ecb50d50100c/third_party/WebKit/LayoutTests/fast/dom/Window/property-access-on-cached-properties-after-frame-navigated-expected.txt
[modify] https://crrev.com/81dd83a5bae93da969e909c58ed3ecb50d50100c/third_party/WebKit/LayoutTests/fast/dom/Window/property-access-on-cached-properties-after-frame-removed-and-gced-expected.txt
[modify] https://crrev.com/81dd83a5bae93da969e909c58ed3ecb50d50100c/third_party/WebKit/LayoutTests/fast/dom/Window/property-access-on-cached-properties-after-frame-removed-expected.txt
[modify] https://crrev.com/81dd83a5bae93da969e909c58ed3ecb50d50100c/third_party/WebKit/LayoutTests/fast/dom/Window/property-access-on-cached-window-after-frame-navigated-expected.txt
[modify] https://crrev.com/81dd83a5bae93da969e909c58ed3ecb50d50100c/third_party/WebKit/LayoutTests/fast/dom/Window/property-access-on-cached-window-after-frame-removed-and-gced-expected.txt
[modify] https://crrev.com/81dd83a5bae93da969e909c58ed3ecb50d50100c/third_party/WebKit/LayoutTests/fast/dom/Window/property-access-on-cached-window-after-frame-removed-expected.txt
[modify] https://crrev.com/81dd83a5bae93da969e909c58ed3ecb50d50100c/third_party/WebKit/LayoutTests/fast/dom/Window/resources/window-property-collector.js
[add] https://crrev.com/81dd83a5bae93da969e909c58ed3ecb50d50100c/third_party/WebKit/LayoutTests/fast/dom/viewport/viewport-dimensions-iframe.html
[modify] https://crrev.com/81dd83a5bae93da969e909c58ed3ecb50d50100c/third_party/WebKit/Source/core/core.gypi
[add] https://crrev.com/81dd83a5bae93da969e909c58ed3ecb50d50100c/third_party/WebKit/Source/core/frame/DOMVisualViewport.cpp
[add] https://crrev.com/81dd83a5bae93da969e909c58ed3ecb50d50100c/third_party/WebKit/Source/core/frame/DOMVisualViewport.h
[modify] https://crrev.com/81dd83a5bae93da969e909c58ed3ecb50d50100c/third_party/WebKit/Source/core/frame/DOMWindow.h
[modify] https://crrev.com/81dd83a5bae93da969e909c58ed3ecb50d50100c/third_party/WebKit/Source/core/frame/LocalDOMWindow.cpp
[modify] https://crrev.com/81dd83a5bae93da969e909c58ed3ecb50d50100c/third_party/WebKit/Source/core/frame/LocalDOMWindow.h
[modify] https://crrev.com/81dd83a5bae93da969e909c58ed3ecb50d50100c/third_party/WebKit/Source/core/frame/VisualViewport.h
[modify] https://crrev.com/81dd83a5bae93da969e909c58ed3ecb50d50100c/third_party/WebKit/Source/core/frame/VisualViewport.idl
[modify] https://crrev.com/81dd83a5bae93da969e909c58ed3ecb50d50100c/third_party/WebKit/Source/core/frame/Window.idl