After selecting item in NSPopUpButtonCell inside custom NSCell table view must be refocused

Question

I have an NSPopUpButtonCell inside a custom NSCell which is being used in an NSTableView.

I'm using

- (BOOL)startTrackingAt:(NSPoint)startPoint inView:(NSView *)controlView

to pop up the pop up menu when it is required.

The problem is that after an item is selected from the menu the table view seems to lose focus, such that it must be clicked twice to select another row, etc (once to refocus it and once to select the new row).

Can anybody explain to me why this is and how I can fix it?