Using the latest Emacs26 built from master on MacOS Sierra using the macOS
window system, I see the following if I have two frames: f1 and f2.

I start with the mouse within a window of f1 and selected frame of f1;
(mouse-position) also reports f1.

I click mouse-1 in an Emacs window of f2 and leave it there.
  (selected-frame) is now f2
  macOS window manager highlights f2 as active window
  (frame-focus) returns nil
  (mouse-position) reports a frame of f1       <<< WRONG


I click mouse-1 again in the same Emacs window of f2.
  (selected-frame) is still f2
  macOS window manager highlights f2 as active window
  (frame-focus) returns nil
  (mouse-position) now reports a frame of f2

This happens consistently in testing.  This must be a bug in mouse-position
for macOS, right?  Why would (mouse-position) still report f1 when f2 is
the selected frame?  Maybe this is why I am seeing the wrong frame on drag
releases too.

If anyone else could confirm this and whether it happens on other window
systems that use a mouse click to select windows, that would be helpful.

Bob

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