Tentatively assigning to pbos@ as he worked with spacing previously.

Not a regression on Windows 10 Canary 69.0.3464.0. This behavior doesn't make sense to me unless features::kViewsBrowserWindows is disabled (but you're obviously using a views window), the layout constants used by ToolbarActionsBar should be the same on all platforms.

+ Mac folks as an initial shot in the dark. Any chance that the views window is turned on in a different way so that this feature is no longer enabled? Related code check: https://cs.chromium.org/chromium/src/chrome/browser/ui/toolbar/toolbar_actions_bar.cc?l=141&rcl=e17296f164793f5797524fdbbe312823e64b5cd9

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Maybe interesting: I see the issue with the 0px spacing between the extension icons on a device with latest macOS 10.13.4, but on another device with macOS 10.12.6 the spacing looks fine with 4px between the icons.

Maybe macOS 10.13.x-specific?

This the behavior on macOS 10.12: 

1.) Spacing okay
2.) CMD-Q to close the window
3.) Restart Canary -> Spacing is broken
4.) CMD-Q to close the window
5.) Restart Canary -> Spacing is okay
6.) CMD-Q to close the window
7.) Restart Canary -> Spacing is broken
8.) CMD-Q to close the window
9.) Restart Canary -> Spacing is okay

and so on... this is really strange :-(

A screencast is attached.

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Same behavior on macOS 10.13.4 as described in c#4.

Thanks! I'll take this since it may be fixed by https://chromium-review.googlesource.com/c/chromium/src/+/1091416 (if not now, a later patchset)

Is this fixed?

Can not repro anylonger - I think this was fixed in  issue 859048 , which was a similar bug.

Marking fixed then. Thanks!