If I revert page zoom to 100%, the overflow disappears.  So it is almost certainly related to that setting.

This is happening for both extensionview and the new WebUI controller.

bettes@, what would be the expected behavior here? Should we make the dialog wider (I fear the dialog would get too wide if the zoom ratio is high), or should we make the layout adaptive (e.g. make sliders get shorter at higher zoom ratio) to make it fit?

It's problematic that on Mac, the horizontal scroll bar is hidden until you scroll, so it's not obvious that you can scroll horizontally to reveal the stop button.

I tried to repro this on Linux 62.0.3202.38 (with varying zoom levels) and at first it looked to be working fine.  I also tried increasing the font size (from chrome://settings) and that also worked fine.

However, when I combined them (large font and increasing zoom levels) then I was able to repro a similar effect on Linux as well.

Is there a way for us to not respect the zoom level in the MR dialog?  I mean you can zoom the tab all you want but other Chrome dialogs (e.g. Print Preview) don't change their size or layouts.  

Have you seen https://chromium-review.googlesource.com/c/chromium/src/+/674179. It sounds related to the problem here.

Thanks for pointing that out.  In this case the browser window was already maximized.

Re: skonig@

We can try do that, but it could be technically complicated, and poses accessibility concerns.  Ideally users should be able to increase the zoom level to make text more comfortable to read, or to be able to read it at all.  I would say the behavior of the print preview dialog is a bug, not a feature.

mfoltz: We talked with takumif@ over email, the observed Print Preview behavior is:

1) Zooming (ctrl + +)  while Print Preview is open zooms the page content, while the Print Preview dialog stays the same.
2) Changing the zoom setting does not affect Print Preview dialogs.
3) Changing the text size setting does affect newly created Print Preview dialogs, but not existing ones.

The issue here is (2) does affect the Cast dialog.

Accessibility is certainly a concern, but I should point out that chrome://settings is for controlling the appearance of web contents, not browser UI. e.g. the Omnibox does not change with the zoom or text size settings. Print Preview and Cast are browser UI. It just happens that they are implemented with WebUI, but that's an implementation detail. So a separate issue is (3) should not affect Print Preview and Cast.

Found that the relevant code is in PrintPreviewDialogController::CreatePrintPreviewDialog at [1]. I verified in a debug build that if you remove those lines, the preview dialog is affected by the default zoom. There is no equivalent clearing of zoom in MediaRouterDialogControllerImpl::CreateMediaRouterDialog (see [2]).

[1] https://cs.chromium.org/chromium/src/chrome/browser/printing/print_preview_dialog_controller.cc?l=373
[2] https://cs.chromium.org/chromium/src/chrome/browser/ui/webui/media_router/media_router_dialog_controller_impl.cc?l=234

rbpotter@, thestig@ and dpapad@, thank you so much for looking into this!

I'll be preparing a CL with the relevant snippet that rbpotter@ mentioned.

re: comment 9 - nice find.
re: commnet 10 - happy to review if the CL is ready.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/784333ab6389500141c66786c226b8dd1a620179

commit 784333ab6389500141c66786c226b8dd1a620179
Author: Takumi Fujimoto <takumif@chromium.org>
Date: Thu Nov 02 20:42:31 2017

[Media Router Dialog] Reset the zoom level of MR dialog contents

This prevents the page zoom in chrome://settings from affecting the dialog.
This affects the WebUI route controller but not extensionview.

Bug:  774856 
Change-Id: I991d2c6d11e283e1ec2276f0e08287400fc0c1fb
Reviewed-on: https://chromium-review.googlesource.com/744563
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Derek Cheng <imcheng@chromium.org>
Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513598}
[modify] https://crrev.com/784333ab6389500141c66786c226b8dd1a620179/chrome/browser/ui/webui/media_router/media_router_dialog_controller_impl.cc