I think there are three bugs on the macOS side here: 1. On windows that have titlebarAppearsTransparent set to YES, the titlebar separator should never be drawn. (But in this testcase it is.) 2. The floating full screen titlebar window should adopt the titlebarSeparatorStyle that was specified on the original window. (But it doesn't.) 3. When the separator is drawn on the floating full screen titlebar window, it's drawn as a glitchy white line. (instead of a dark gray separator)
Bug 1700211 Comment 9 Edit History
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I think there are three bugs on the macOS side here: 1. On windows that have titlebarAppearsTransparent set to YES, the titlebar separator should never be drawn - the value of titlebarSeparatorStyle shouldn't matter. 2. The floating full screen titlebar window should adopt the titlebarSeparatorStyle that was specified on the original window. 3. When the separator is drawn on the floating full screen titlebar window, it's drawn as a glitchy white line.