@amp - any ideas?

I would think this has more to do with the sink queries and source urn's then the actual capturing part of it.

Derek or Brandon should have more context on that side.  I'm pretty sure we send out two queries (one for tab and one for desktop)and that each provider then needs to return availability as true for that query in order for it to be marked as supported (after which the MR UI checks supported modes to choose the source shown).

It sounds like the desktop query is returning true, but the tab capture one is not (or is late).  But I'm just guessing.

Brandon, does this ring any bells?

It looks like this was caused by crrev.com/2221933003.  The root cause seems to be that polymer doesn't always render the list after we update the sinks but I don't know why that would only show up now.  I have a fix out for review here: https://codereview.chromium.org/2252313002/

I'm seeing this in M53 as well, can we merge the fix back to M53?

Chrome: 53.0.2785.70
Plat/ OS: Win 8

Yes, it looks like the fix I have works on M53 as well.

Please request a merge to M53 as soon as fix is baked/verified in Canary and it is a safe merge. 

@btolsch: Gentle Ping.

Could you please provide an update on this issue as per the comment #9

Thank you.

A friendly reminder that M53 Stable is launching VERY soon! Your bug is labelled as Stable ReleaseBlock, pls make sure to land the fix and get it merged into the release branch ASAP (before 5:00 PM PT, Tuesday) so we can take it for this week LAST Beta release for Desktop. Thank you!

Note: Merge has to happen by Friday, August 26th, 5:00 PM PST in order to make into the desktop Stable final build cut. 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7e3c3e80f15d513f9c2fe7a59b1854c7e1ff47e9

commit 7e3c3e80f15d513f9c2fe7a59b1854c7e1ff47e9
Author: btolsch <btolsch@chromium.org>
Date: Mon Aug 22 23:15:24 2016

[Media Router WebUI] Use updated sink on click

Since crrev.com/2221933003, the dom-repeat element that creates the sink
list doesn't seem to correctly update its element-item mapping.  Instead
we will use the most current sink object with the same ID in our own
map.

BUG= 637222 
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2252313002
Cr-Commit-Position: refs/heads/master@{#413573}

[modify] https://crrev.com/7e3c3e80f15d513f9c2fe7a59b1854c7e1ff47e9/chrome/browser/resources/media_router/elements/media_router_container/media_router_container.js

verified this on today's canary 54.0.2837.0

Your change meets the bar and is auto-approved for M53 (branch: 2785)

Please merge your change to M53 branch 2785 asap or latest by Friday, August 26th, 5:00 PM PST in order to make into the desktop Stable final build cut. Thank you.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6ecece00c2f3b6736fdd4d9a4d92313fa718ad8d

commit 6ecece00c2f3b6736fdd4d9a4d92313fa718ad8d
Author: mark a. foltz <mfoltz@chromium.org>
Date: Thu Aug 25 18:52:45 2016

[Media Router WebUI] Use updated sink on click

Since crrev.com/2221933003, the dom-repeat element that creates the sink
list doesn't seem to correctly update its element-item mapping.  Instead
we will use the most current sink object with the same ID in our own
map.

BUG= 637222 
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2252313002
Cr-Commit-Position: refs/heads/master@{#413573}
(cherry picked from commit 7e3c3e80f15d513f9c2fe7a59b1854c7e1ff47e9)

Review URL: https://codereview.chromium.org/2278723006 .

Cr-Commit-Position: refs/branch-heads/2785@{#751}
Cr-Branched-From: 68623971be0cfc492a2cb0427d7f478e7b214c24-refs/heads/master@{#403382}

[modify] https://crrev.com/6ecece00c2f3b6736fdd4d9a4d92313fa718ad8d/chrome/browser/resources/media_router/elements/media_router_container/media_router_container.js

Per pbommana@'s request - verified for 53.0.2785.89 release.