I believe it's on purpose because it's centered based on the bottom bar and we don't want to move the spinner when the bar shows up. Assigning to steimel@ to triage.

"it's on purpose"

If we want to make a spinner that doesn't move, stick it where it won't clash with another part of the UI.

+amyroberts@ (UX)

The media controls have similar polish issues; bug 877506.

FYI, it also wobbles and that should be fixed too.

Probably worth fixing in 71.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/53e727daf255777aa6ad6feb3d8188ec70db52e0

commit 53e727daf255777aa6ad6feb3d8188ec70db52e0
Author: Tommy Steimel <steimel@chromium.org>
Date: Mon Oct 15 00:08:29 2018

[Media Controls] Fix the size of the overlay play button as 72x72px

This CL removes the code that dynamically sets the overlay play button
size based on the video size. Instead, this just sets the button at
72px by 72px. This solves *most* cases of a bug where the loading
spinner was slightly off-center on certain sizes.

Bug:  894866 , 877113
Change-Id: I96ccda8921ffddddf009fa0b7ee28907201a966f
Reviewed-on: https://chromium-review.googlesource.com/c/1278860
Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599547}
[delete] https://crrev.com/eca9fd593b12dc8111ac2d779ba42df78e872371/third_party/WebKit/LayoutTests/media/controls/overlay-play-button-document-move-expected.txt
[modify] https://crrev.com/53e727daf255777aa6ad6feb3d8188ec70db52e0/third_party/WebKit/LayoutTests/media/controls/overlay-play-button-document-move.html
[delete] https://crrev.com/eca9fd593b12dc8111ac2d779ba42df78e872371/third_party/WebKit/LayoutTests/media/controls/overlay-play-button-narrow-expected.txt
[delete] https://crrev.com/eca9fd593b12dc8111ac2d779ba42df78e872371/third_party/WebKit/LayoutTests/media/controls/overlay-play-button-resizes-with-video.html
[modify] https://crrev.com/53e727daf255777aa6ad6feb3d8188ec70db52e0/third_party/blink/renderer/modules/media_controls/media_controls_impl.cc
[modify] https://crrev.com/53e727daf255777aa6ad6feb3d8188ec70db52e0/third_party/blink/renderer/modules/media_controls/media_controls_impl.h
[modify] https://crrev.com/53e727daf255777aa6ad6feb3d8188ec70db52e0/third_party/blink/renderer/modules/media_controls/resources/modernMediaControls.css

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a8732928a5d9c25279eb4bfe611e0fccf5e21cea

commit a8732928a5d9c25279eb4bfe611e0fccf5e21cea
Author: Tommy Steimel <steimel@chromium.org>
Date: Tue Oct 16 16:07:13 2018

[Media Controls] Fix the size of the overlay play button as 72x72px

This CL removes the code that dynamically sets the overlay play button
size based on the video size. Instead, this just sets the button at
72px by 72px. This solves *most* cases of a bug where the loading
spinner was slightly off-center on certain sizes.

Bug:  894866 , 877113
Change-Id: I96ccda8921ffddddf009fa0b7ee28907201a966f
Reviewed-on: https://chromium-review.googlesource.com/c/1278860
Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#599547}(cherry picked from commit 53e727daf255777aa6ad6feb3d8188ec70db52e0)
Reviewed-on: https://chromium-review.googlesource.com/c/1283552
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/branch-heads/3578@{#42}
Cr-Branched-From: 4226ddf99103e493d7afb23a4c7902ee496108b6-refs/heads/master@{#599034}
[delete] https://crrev.com/6bfd5223ba5bed7168b2bfe92c6da0acaf976e8b/third_party/WebKit/LayoutTests/media/controls/overlay-play-button-document-move-expected.txt
[modify] https://crrev.com/a8732928a5d9c25279eb4bfe611e0fccf5e21cea/third_party/WebKit/LayoutTests/media/controls/overlay-play-button-document-move.html
[delete] https://crrev.com/6bfd5223ba5bed7168b2bfe92c6da0acaf976e8b/third_party/WebKit/LayoutTests/media/controls/overlay-play-button-narrow-expected.txt
[delete] https://crrev.com/6bfd5223ba5bed7168b2bfe92c6da0acaf976e8b/third_party/WebKit/LayoutTests/media/controls/overlay-play-button-resizes-with-video.html
[modify] https://crrev.com/a8732928a5d9c25279eb4bfe611e0fccf5e21cea/third_party/blink/renderer/modules/media_controls/media_controls_impl.cc
[modify] https://crrev.com/a8732928a5d9c25279eb4bfe611e0fccf5e21cea/third_party/blink/renderer/modules/media_controls/media_controls_impl.h
[modify] https://crrev.com/a8732928a5d9c25279eb4bfe611e0fccf5e21cea/third_party/blink/renderer/modules/media_controls/resources/modernMediaControls.css

The following revision refers to this bug: 
https://chromium.googlesource.com/chromium/src.git/+/a8732928a5d9c25279eb4bfe611e0fccf5e21cea

Commit: a8732928a5d9c25279eb4bfe611e0fccf5e21cea
Author: steimel@chromium.org
Commiter: steimel@chromium.org
Date: 2018-10-16 16:07:13 +0000 UTC

[Media Controls] Fix the size of the overlay play button as 72x72px

This CL removes the code that dynamically sets the overlay play button
size based on the video size. Instead, this just sets the button at
72px by 72px. This solves *most* cases of a bug where the loading
spinner was slightly off-center on certain sizes.

Bug:  894866 , 877113
Change-Id: I96ccda8921ffddddf009fa0b7ee28907201a966f
Reviewed-on: https://chromium-review.googlesource.com/c/1278860
Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#599547}(cherry picked from commit 53e727daf255777aa6ad6feb3d8188ec70db52e0)
Reviewed-on: https://chromium-review.googlesource.com/c/1283552
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/branch-heads/3578@{#42}
Cr-Branched-From: 4226ddf99103e493d7afb23a4c7902ee496108b6-refs/heads/master@{#599034}

Is there anything else to do here?