That is because of the Harmony dialog width snapping. I am not sure if it is intentional per se, but it is the width snapping performing as designed. The dialog's natural bounds on my Linux system are 352x129, which we snap up to 448x129. I will ask the designers about this dialog.

The root cause why the dialog is 352x129 is this: https://cs.chromium.org/chromium/src/chrome/browser/ui/views/permission_bubble/permission_prompt_impl.cc?l=309

it sets the PermissionsBubbleDialogDelegateView's width to 320, then we add the usual bubble margins, get 352, and round up to 448, so that's bad. If I remove that, though, the bubble collapses down to a tiny size in non-Harmony mode, because the title is wide and the permission strings are narrow. Awkward.

A bubble won't know how wide it is until it's in a Widget, and GetPreferredSize() needs to work before there is a Widget, so the fragment,

gfx::Size PermissionsBubbleDialogDelegateView::GetPreferredSize() const {
  // TODO(estade): bubbles should default to this width.
  const int kWidth = 320 - GetInsets().width();
  return gfx::Size(kWidth, GetHeightForWidth(kWidth));
}

won't do the right thing before or after Harmony.

We can, perhaps, put a helper on BubbleDialogDelegateView with a hard-coded constant that provides the additional width that the default NonClientFrameView will add to the edges of bubbles. That should help bubbles transition over to harmony before they all get a useful default minimum width.

But also it seems reasonable just to impose the min width of 320 when ShouldSnapFrameWidth() returns true pre-harmony as well. That will let us just delete some of these GetPreferredSize() overrides.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/436d5e22db0b978fc69612f62f8a9432622d8fe1

commit 436d5e22db0b978fc69612f62f8a9432622d8fe1
Author: ellyjones <ellyjones@chromium.org>
Date: Wed May 31 13:59:07 2017

views: impose minimum width on non-Harmony snappables

Some dialogs, like the permission bubble, have very narrow
preferred contents view widths, but titles that require wider
dialogs. Instead of these dialogs individually imposing
minimum widths, require all snappable non-Harmony dialogs to
have a minimum width.

BUG= 726184 

Review-Url: https://codereview.chromium.org/2907873002
Cr-Commit-Position: refs/heads/master@{#475900}

[modify] https://crrev.com/436d5e22db0b978fc69612f62f8a9432622d8fe1/chrome/browser/ui/views/permission_bubble/permission_prompt_impl.cc
[modify] https://crrev.com/436d5e22db0b978fc69612f62f8a9432622d8fe1/ui/views/layout/layout_provider.cc

This got fixed a while ago by 12d3545cdca172bf6e8f1fb7e831a2d12c916f26.