Just in case this hasn't been caught yet:

In "(elisp)Key Sequence Input" near the end of the node is this:

  For example, if you call ‘read-key-sequence’ and then click the mouse
on the window’s mode line, you get two events, like this:
                                   ^^^^^^^^^^

     (read-key-sequence "Click on the mode line: ")
          ⇒ [mode-line
              (mouse-1
               (#<window 6 on NEWS> mode-line
                (40 . 63) 5959987))]
----

But only one event is shown.

Bob