https://jsfiddle.net/b9nhrab0/ maybe. jsbin expires. Hope jsfiddle doesn't.

Test page at http://avidrissman.github.io/htmltests/delay.html .

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/01c32ae1f647e38c54b9978260a6d126698745df

commit 01c32ae1f647e38c54b9978260a6d126698745df
Author: Avi Drissman <avi@chromium.org>
Date: Tue Jul 25 15:59:03 2017

Make the TabStripModelBridge a protocol.

This is just cleanup.

BUG= 747117 

Change-Id: If15f31d8c16bf70e57cb5fab39886878bf742889
Reviewed-on: https://chromium-review.googlesource.com/585167
Reviewed-by: Leonard Grey <lgrey@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#489319}
[modify] https://crrev.com/01c32ae1f647e38c54b9978260a6d126698745df/chrome/browser/ui/cocoa/tabs/tab_strip_controller.h
[modify] https://crrev.com/01c32ae1f647e38c54b9978260a6d126698745df/chrome/browser/ui/cocoa/tabs/tab_strip_controller.mm
[modify] https://crrev.com/01c32ae1f647e38c54b9978260a6d126698745df/chrome/browser/ui/cocoa/tabs/tab_strip_model_observer_bridge.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7b9c441903a7c6109fb23837f81ba5508c2dc78e

commit 7b9c441903a7c6109fb23837f81ba5508c2dc78e
Author: Avi Drissman <avi@chromium.org>
Date: Wed Jul 26 16:38:44 2017

Use ObjCCast in TabStripController.

This is just clean-up; no functional change intended.

BUG= 747117 

Change-Id: Icf562942ae45efddb721e90d1b3c81d2967e1034
Reviewed-on: https://chromium-review.googlesource.com/586643
Reviewed-by: Leonard Grey <lgrey@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#489666}
[modify] https://crrev.com/7b9c441903a7c6109fb23837f81ba5508c2dc78e/chrome/browser/ui/cocoa/tabs/tab_strip_controller.mm

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fb91c4fd4d2544c08d25422f77282ec84184e238

commit fb91c4fd4d2544c08d25422f77282ec84184e238
Author: Avi Drissman <avi@chromium.org>
Date: Wed Jul 26 17:54:22 2017

Create a UI state of "tab needs attention".

This is a variation on the UI indication that a pinned tab has changed its title, but is triggerable from the browser.

BUG= 747117 

Change-Id: Ibda92eb1dd709e989b5de12f8d9b61188294649b
Reviewed-on: https://chromium-review.googlesource.com/585302
Commit-Queue: Avi Drissman <avi@chromium.org>
Reviewed-by: Leonard Grey <lgrey@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#489688}
[modify] https://crrev.com/fb91c4fd4d2544c08d25422f77282ec84184e238/chrome/browser/ui/cocoa/tabs/tab_controller.h
[modify] https://crrev.com/fb91c4fd4d2544c08d25422f77282ec84184e238/chrome/browser/ui/cocoa/tabs/tab_controller.mm
[modify] https://crrev.com/fb91c4fd4d2544c08d25422f77282ec84184e238/chrome/browser/ui/cocoa/tabs/tab_strip_controller.mm
[modify] https://crrev.com/fb91c4fd4d2544c08d25422f77282ec84184e238/chrome/browser/ui/cocoa/tabs/tab_strip_model_observer_bridge.h
[modify] https://crrev.com/fb91c4fd4d2544c08d25422f77282ec84184e238/chrome/browser/ui/cocoa/tabs/tab_strip_model_observer_bridge.mm
[modify] https://crrev.com/fb91c4fd4d2544c08d25422f77282ec84184e238/chrome/browser/ui/cocoa/tabs/tab_view.h
[modify] https://crrev.com/fb91c4fd4d2544c08d25422f77282ec84184e238/chrome/browser/ui/cocoa/tabs/tab_view.mm
[modify] https://crrev.com/fb91c4fd4d2544c08d25422f77282ec84184e238/chrome/browser/ui/cocoa/tabs/tab_view_unittest.mm
[modify] https://crrev.com/fb91c4fd4d2544c08d25422f77282ec84184e238/chrome/browser/ui/tabs/tab_strip_model.cc
[modify] https://crrev.com/fb91c4fd4d2544c08d25422f77282ec84184e238/chrome/browser/ui/tabs/tab_strip_model.h
[modify] https://crrev.com/fb91c4fd4d2544c08d25422f77282ec84184e238/chrome/browser/ui/tabs/tab_strip_model_observer.cc
[modify] https://crrev.com/fb91c4fd4d2544c08d25422f77282ec84184e238/chrome/browser/ui/tabs/tab_strip_model_observer.h
[modify] https://crrev.com/fb91c4fd4d2544c08d25422f77282ec84184e238/chrome/browser/ui/views/tabs/browser_tab_strip_controller.cc
[modify] https://crrev.com/fb91c4fd4d2544c08d25422f77282ec84184e238/chrome/browser/ui/views/tabs/browser_tab_strip_controller.h
[modify] https://crrev.com/fb91c4fd4d2544c08d25422f77282ec84184e238/chrome/browser/ui/views/tabs/tab.cc
[modify] https://crrev.com/fb91c4fd4d2544c08d25422f77282ec84184e238/chrome/browser/ui/views/tabs/tab.h
[modify] https://crrev.com/fb91c4fd4d2544c08d25422f77282ec84184e238/chrome/browser/ui/views/tabs/tab_strip.cc
[modify] https://crrev.com/fb91c4fd4d2544c08d25422f77282ec84184e238/chrome/browser/ui/views/tabs/tab_strip.h
[modify] https://crrev.com/fb91c4fd4d2544c08d25422f77282ec84184e238/chrome/browser/ui/views/tabs/tab_strip_unittest.cc

Implementation doc is https://docs.google.com/document/d/1TyvPRhAd5yjbXHCiVg_u0NLBxiBEywmcA3b3xgtgWbc/

Instead of returning immediately does it make sense to have a Promise here? Otherwise won't any data transmitted to the prompt be lost in a background tab since the alert is shown after the call has returned?

> Instead of returning immediately does it make sense to have a Promise here?

No for two reasons.

First is that the spec can't change. This is a legacy API; we're trying to make it behave better within the existing spec.

Second, alert() returns no useful data. You're likely thinking of confirm().

> Otherwise won't any data transmitted to the prompt be lost in a background tab since the alert is shown after the call has returned?

alert() returns no useful data, so that's not a concern.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/66b55f820b6deee6c7d81296f9d63b3d596180a7

commit 66b55f820b6deee6c7d81296f9d63b3d596180a7
Author: Avi Drissman <avi@chromium.org>
Date: Thu Oct 26 19:59:27 2017

Remove activation from alert dialogs.

BUG= 747117 

Change-Id: I4c7a4b75465ceca2eb79a8fb61e4196ec5e4910e
Reviewed-on: https://chromium-review.googlesource.com/720176
Reviewed-by: Nick Carter <nick@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511934}
[modify] https://crrev.com/66b55f820b6deee6c7d81296f9d63b3d596180a7/chrome/browser/ui/blocked_content/popup_blocker_browsertest.cc
[modify] https://crrev.com/66b55f820b6deee6c7d81296f9d63b3d596180a7/chrome/browser/ui/browser_browsertest.cc
[modify] https://crrev.com/66b55f820b6deee6c7d81296f9d63b3d596180a7/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc
[modify] https://crrev.com/66b55f820b6deee6c7d81296f9d63b3d596180a7/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.h

Ok thanks for fielding my question. There is a lot to understand about this topic.

 Issue 913903  has been merged into this issue.