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What happens when you have the edit panel displayed as a pop up rather than docked to the main panel, what happens with the button then? I really don't see the need for it, you can already right click to access it or set up a command key or use the proper shortcut.
It is just opened/closed as configured. Nothing happens with the button.
Well, that doesn't match the UI either.
Currently, if Track & Artist Info is docked in the panel, the open/close icon is there, but if it's a tab, the icon doesn't stay there and open and close the tab.
click a button in a nonstandard place in order to open/close a panel instead of having a button where it is expected.
Where do you propose that it is expected to be?
If the Edit and Track&Info are both docked in the panel, would 2 icons be on the right, or the edit panel open/close go on the left?
IMO the edit panel is an extra panel, like preferences, that is opened when needed. It is not part of the navigation/information panels that form the main UI.
Since the icon doesn't exist, I cant see how it can be a "nonstandard place". Whether it be a button on the AddressBar or an icon on the StatusBar, you have to learn where to 'click' to open the panel.