Window moves when clicking control in title region with titlebarAppearsTransparent

Question

I have a window that uses the Yosemite titlebarAppearsTransparent and titleVisibility = NSWindowTitleHidden properties to extend the content under the title region. It is also movableByWindowBackground.

This works fine, however if I have a control, e.g. NSPopUpButton, in the titlebar region, when I click and attempt to choose a menu item, the whole window moves (while the menu stays put).

Here's a demo:

I've tried subclassing the NSPopUpButton and overriding mouseDownCanMoveWindow, but that doesn't help (it's called, but apparently ignored).

Any ideas?

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Edit: I created a demo project to reproduce the issue. It seems related to embedding the popup inside a NSSplitViewController. I can't avoid that, though, so I'm not sure what the solution might be.

Here's the demo project, if you want to try it.

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Edit 2: apparently it is an OS bug, with no workaround. Filed Radar #21973457.