We need to find an owner of this bug. I am not very familiar with how the viewport is computed. Ilya, Yoav, suggestions?

I was able to fix this for Desktop (the CL is in-flight).

On Android, all the existing APIs that I tried return 1080px on my phone. My phone indeed has 1080 px resolution. However, the Blink code that computes the viewport width computes it as 980 px. Yoav, do you happen to know how 980px is computed?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5c28afb44d6b24345d7f86c7947b8a8b17eb300f

commit 5c28afb44d6b24345d7f86c7947b8a8b17eb300f
Author: Tarun Bansal <tbansal@chromium.org>
Date: Sat Mar 17 02:55:20 2018

Include viewport-width client hint on main frame navigations on desktop

Bug:  821974 
Change-Id: Icceeabb2e31c702341d898193f5ad6870b2d3f1e
Reviewed-on: https://chromium-review.googlesource.com/967095
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543925}
[modify] https://crrev.com/5c28afb44d6b24345d7f86c7947b8a8b17eb300f/chrome/browser/client_hints/client_hints.cc
[modify] https://crrev.com/5c28afb44d6b24345d7f86c7947b8a8b17eb300f/chrome/browser/client_hints/client_hints_browsertest.cc
[modify] https://crrev.com/5c28afb44d6b24345d7f86c7947b8a8b17eb300f/chrome/test/data/client_hints/accept_ch_with_lifetime.html.mock-http-headers
[modify] https://crrev.com/5c28afb44d6b24345d7f86c7947b8a8b17eb300f/chrome/test/data/client_hints/accept_ch_without_lifetime.html.mock-http-headers
[modify] https://crrev.com/5c28afb44d6b24345d7f86c7947b8a8b17eb300f/chrome/test/data/client_hints/accept_ch_without_lifetime_img_localhost.html.mock-http-headers

Apologies, but I don't remember specifics here. If I had to guess, I'd guess it's something that's the result of PageScaleConstraints[1]. So I would start by printing out the values there on Android. (on the roads, so can't build for Android here...)

Thinking about it, `viewport-width` for the navigation request could be different than the one for subresource requests as `<meta name=viewport>` may be impacting the results of what you see inside the renderer.

tbansal@ - is the 980px vs. 1080px difference something you see in all pages? Also, is the viewport-width value on the navigation request specified somehow?

[1] https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/frame/PageScaleConstraintsSet.h?type=cs&sq=package:chromium&l=48

+rune and mounir who probably know more than me about Android viewport settings

Seems like 980px is the default value.
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/css/viewportAndroid.css

https://developers.google.com/web/fundamentals/design-and-ux/responsive/

The developer can override it later by using meta tags, but that can happen only after the browser receives response back from the server. For main frame requests on Android, seems like it should be safe to always send 980px as the value.

OK, that makes sense.
@igrigorik - would be good to specify the browser behavior here somewhere, although I'm not sure what's the best place for that. Maybe Fetch?

Yoav: can you clarify which part(s) you're referring to?

When it comes to Viewport-Width, there are multiple values[1] that can be sent both for the case of navigation requests as well as subresource requests. It makes sense to me that we'd define the navigation request to use the initial viewport and subresource requests to use the resource viewport width.

[1] https://drafts.csswg.org/css-device-adapt/#constraining-defs

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/79df868e50bae74269ced7a1a45510b254281d7b

commit 79df868e50bae74269ced7a1a45510b254281d7b
Author: Tarun Bansal <tbansal@chromium.org>
Date: Tue Mar 20 23:01:36 2018

Send viewport width on main frame navigations on Android

The value is set to 980px which is the default value on Android.

Bug:  821974 
Change-Id: I6f3842d503ba62b752969a97e1e714109afff497
Reviewed-on: https://chromium-review.googlesource.com/971351
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544578}
[modify] https://crrev.com/79df868e50bae74269ced7a1a45510b254281d7b/chrome/browser/client_hints/client_hints.cc
[modify] https://crrev.com/79df868e50bae74269ced7a1a45510b254281d7b/chrome/browser/client_hints/client_hints_browsertest.cc