I closed #34281 as a duplicate of this.  Eric Martin said "After entering a line number, clicking OK brings to the foreground and makes active the first hidden window (in the simplest case where we have just one window for a .py file and the Shell window, the former being in the foreground, clicking OK brings the Shell window to the foreground)."

Normally, one window is active, as indicated by the traffic lights, and one widget has the focus, which for entry widgets is indicated by the cursor.  I tested with 3.7.0 with MacOS 10.13.6 and a shell and editor window.

IDLE > About IDLE: closing either way turns lights on in original window but text does not get focus (no cursor).  Neither TAB nor click return focus, but the window is not frozen either.  Selection, deselection, and copy work, cut and paste do not.

As far as I know, on Mac, cut and paste as well as copy and selection are handled entirely by tk, and not IDLE.  My interpretation is that the window is 'stuck' in an 'illegal' state, partly enabled, partly disabled.  Clicking and activating the other window disables the stuck window, so that clicking on it enables it.

IDLE > Settings works the same.  But...

Options > Configure IDLE works as Eric described above.  Closing with OK or Cancel or Red light activates the *other* window, in a stuck state.
(If there is no other IDLE window, then something else must be clicked to unstick the originally active window.)

Edit > Go to line: If one cancels, behavior is normal.  If one enters a number and clicks OK, the other window is activated (lights on) without a cursor while the cursor goes where requested in the original, apparently 'inactive' window.  And keyboard input goes where the cursor is, in that unlighted window.  AFAIK, this is 'impossible'.