1696465 - With the Proton theme, on macOS the tab bar spacing changes if the window is resized to cover all available area

Status: NEW

(Firefox :: Theme, defect, P3)

Description

[Markus Stange [:mstange]]

Attachment: screen recording

See the screen recording.

Steps to reproduce:

1. Have a Firefox window with some tabs but not enough tabs to cause overflow.
2. Resize the window to cover all available area on the screen.

Expected results:
Tab bar spacing should not change when the window is resized.

Actual results:
Tab bar spacing shifts when the window changes between covering all available area on the screen and being smaller than that.

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I think what happens here is that whenever the window is "zoomed" we set the sizemode of the window to maximized, and then CSS rules react to the maximized state. "Zooming" is the macOS term for what happens when you double-click the titlebar or Option-click the green window button. And a window can become zoomed by manual resizing.
We've been treating "zoomed" as "maximized" on macOS since bug 429954. I think the original motivation was that it lets us remember the non-zoomed size across browser restarts even if the browser is closed while zoomed.

However, on macOS there is no user expectation that windows will look different if they're in zoomed mode vs non-zoomed mode.

I think we have two options:

1. We could ignore the maximized sizemode for styling purposes when we're on macOS.
2. Or, at the platform level, we could stop treating the zoomed state as maximized sizemode.

Option 2 seems more correct but option 1 is lower risk.