Optionally converting windows to tabs is an over-arching goal for IDLE.  To do it with IDLE code, IDLE must create a frame (and keep a reference) but not a toplevel.  I presume that  the Mac window manager extracts the window title and frame from the window, discards the window, and puts the title and frame on a tab.  Kevin, does tcl/tk know about or cooperate with this conversion?

IDLE's default editor size is configured as 40 lines of 80 characters.  On my MacBook Air, the windows are actually 38 lines; that is all that fits between the title bar and taskbar. Additional windows overlap, but are moved to the right.  If I set the window height to less than 38, say 30, the initial window does not fill the vertical space, showing some of the background beneath.  Additional windows appear a bit down as well as to the right.

I found the setting under System Preferences Dock and more or less confirm Ned's report.  It appears that the status bar is hidden rather than deleted.  I set the initial window size to 36, 2 lines less than the actual maximum, hoping the status bar would remain in a larger window after opening a second window.  No such luck. The size remains 36, with the bar hidden.  For the editor windows, grabbing the vertical resize made the status appear and stay.  For Shell, it disappeared either immediately or when tabbing away and back.  Since Shell subclasses EditorWindow, I don't know why the difference.

The resolution to this issue from IDLE's viewpoint should be a new IDLE doc subsection 'IDLE on MacOS', probably under 'Help and Preferences'.  It would explain differences from IDLE on Linux and Windows.  Besides this issue, there is the single title bar, menu differences, and zoom behavior (horizontal expansion as well as vertical).  Also, as far as I can tell so far (and someone correct me if this is wrong), only 1 instance of IDLE can run (for a given binary, at least), which means only 1 Shell.  (On Windows, I can run multiple instances of a given binary, and I presume the same is true on non-Mac *nix.)