The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2793a25de466de7fc9d9d390b87860207b3865d8

commit 2793a25de466de7fc9d9d390b87860207b3865d8
Author: Peter Boström <pbos@chromium.org>
Date: Mon May 21 23:06:52 2018

Narrow extension-uninstall dialog as a bubble

This dialog can be rendered both as a modal dialog and a popover, the
size of it needs to reflect both visual styles and use the appropriate
width constant.

Bug:  chromium:843624 
Change-Id: I2c69a1704bb6502d55115078486691930bc9d74e
Reviewed-on: https://chromium-review.googlesource.com/1062910
Commit-Queue: Bret Sepulveda <bsep@chromium.org>
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560395}
[modify] https://crrev.com/2793a25de466de7fc9d9d390b87860207b3865d8/chrome/browser/ui/views/extensions/extension_uninstall_dialog_view.cc

Tested this issue on Windows 10, Mac OS 10.13.3 and Ubuntu 14.04 on the build without fix 68.0.3430.0 and on the latest Canary 68.0.3437.2 as per the original comment.

Can see the narrow extension uninstall dialog on the latest Canary on Windows and Linux, but on Mac OS it is same as the previous versions.
Attached are the screen shots for reference.

pbos@ Request you to check and confirm the fix on the latest Canary 68.0.3437.2 build.

Thanks..

Deleted: 843624-build-without-fix-windows.png 87.3 KB

	843624-build-without-fix-windows.png 87.3 KB View Download

Deleted: 843624-M68-CL.png 90.9 KB

	843624-M68-CL.png 90.9 KB View Download

Deleted: 843624-Mac.png 105 KB

	843624-Mac.png 105 KB View Download

// Linux screenshot

Deleted: 843624-linux.png 214 KB

	843624-linux.png 214 KB View Download

I assume the Mac one is using Cocoa which is why the dialog is not anchoring to the extension. This is a separate implementation and Mac is moving over to the common one. Working as intended. Thanks for checking!