I can reproduce this on Win7 using the following (very strange) steps, which involve chrome remote desktop. I am using i3 which I believe might be significant because the windows (meta/mod) key is used to switch workspaces but also interacts with the remote Win7 session.

1. Connect to the Win7 machine from a linux machine using chrome remote desktop.
2. Open chrome and resize the window so that the sink list in the MR dialog will have to scroll.
3. Switch to another workspace (mod+1).
4. Switch back to the CRD workspace.
5. Launch the MR dialog through the right-click context menu.

Can you grab the Win10 lab device and see if you can repro it with more straight forward steps? The ones you outlined in #4 are... very specific and through CRD.

I think I can repro it by just casting to a cloud device while on our lab network.

Unable to repro this on canary on the same laptop.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/39dfd062d8014833796a544e4b778ed811b98325

commit 39dfd062d8014833796a544e4b778ed811b98325
Author: btolsch <btolsch@chromium.org>
Date: Thu May 26 18:48:50 2016

[Media Router WebUI] Delay putSearchAtBottom_() until after imports

This change ensures that a call to putSearchAtBottom_() is made after
all the html imports have been completed. This should ensure that the
reference to search.offsetHeight is not zero when it sets margins and
padding.

BUG= 614209 ,  609350 
R=apacible@chromium.org
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2008393002
Cr-Commit-Position: refs/heads/master@{#396244}

[modify] https://crrev.com/39dfd062d8014833796a544e4b778ed811b98325/chrome/browser/resources/media_router/elements/media_router_container/media_router_container.js

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b900e1e230e10a966694c3d4ccff40239d7c9301

commit b900e1e230e10a966694c3d4ccff40239d7c9301
Author: btolsch <btolsch@chromium.org>
Date: Wed Jun 01 23:04:11 2016

[Media Router WebUI] Delay putSearchAtBottom_() until after imports

This change ensures that a call to putSearchAtBottom_() is made after
all the html imports have been completed. This should ensure that the
reference to search.offsetHeight is not zero when it sets margins and
padding.

BUG= 614209 ,  609350 
R=apacible@chromium.org
NOTRY=true
NOPRESUBMIT=true

Review-Url: https://codereview.chromium.org/2008393002
Cr-Commit-Position: refs/heads/master@{#396244}
(cherry picked from commit 39dfd062d8014833796a544e4b778ed811b98325)

Review-Url: https://codereview.chromium.org/2028373002
Cr-Commit-Position: refs/branch-heads/2743@{#180}
Cr-Branched-From: 2b3ae3b8090361f8af5a611712fc1a5ab2de53cb-refs/heads/master@{#394939}

[modify] https://crrev.com/b900e1e230e10a966694c3d4ccff40239d7c9301/chrome/browser/resources/media_router/elements/media_router_container/media_router_container.js