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