As I tried to educate myself on what double clicks should really mean I came across this beautiful piece of documentation for elisp (emacs lisp).
Its idea of order is somewhat at odds with what I've seen here and in squeak. Still it makes a lot of sense to me and it includes distinctions for double-drag. I like it because it pleases my sense of consistency and completeness.
elisp doc on repeat mouse events:
21.6.7 Repeat Events
If you press the same mouse button more than once in quick succession without moving the mouse, Emacs generates special repeat mouse events for the second and subsequent presses.
The most common repeat events are double-click events. Emacs generates a double-click event when you click a button twice; the event happens when you release the button (as is normal for all click events).
The event type of a double-click event contains the prefix `double-'. Thus, a double click on the second mouse button with meta held down comes to the Lisp program as M-double-mouse-2. If a double-click event has no binding, the binding of the corresponding ordinary click event is used to execute it. Thus, you need not pay attention to the double click feature unless you really want to.
When the user performs a double click, Emacs generates first an ordinary click event, and then a double-click event. Therefore, you must design the command binding of the double click event to assume that the single-click command has already run. It must produce the desired results of a double click, starting from the results of a single click.
This is convenient, if the meaning of a double click somehow "builds on" the meaning of a single click--which is recommended user interface design practice for double clicks.
If you click a button, then press it down again and start moving the mouse with the button held down, then you get a double-drag event when you ultimately release the button. Its event type contains `double-drag' instead of just `drag'. If a double-drag event has no binding, Emacs looks for an alternate binding as if the event were an ordinary drag.
Before the double-click or double-drag event, Emacs generates a double-down event when the user presses the button down for the second time. Its event type contains `double-down' instead of just `down'. If a double-down event has no binding, Emacs looks for an alternate binding as if the event were an ordinary button-down event. If it finds no binding that way either, the double-down event is ignored.
To summarize, when you click a button and then press it again right away, Emacs generates a down event and a click event for the first click, a double-down event when you press the button again, and finally either a double-click or a double-drag event.
If you click a button twice and then press it again, all in quick succession, Emacs generates a triple-down event, followed by either a triple-click or a triple-drag. The event types of these events contain `triple' instead of `double'. If any triple event has no binding, Emacs uses the binding that it would use for the corresponding double event.
If you click a button three or more times and then press it again, the events for the presses beyond the third are all triple events. Emacs does not have separate event types for quadruple, quintuple, etc. events. However, you can look at the event list to find out precisely how many times the button was pressed.
Function: event-click-count event
This function returns the number of consecutive button presses that led up to event. If event is a double-down, double-click or double-drag event, the value is 2. If event is a triple event, the value is 3 or greater. If event is an ordinary mouse event (not a repeat event), the value is 1.
To generate repeat events, successive mouse button presses must be at approximately the same screen position. The value of double-click-fuzz specifies the maximum number of pixels the mouse may be moved between two successive clicks to make a double-click.
To generate repeat events, the number of milliseconds between successive button presses must be less than the value of double-click-time. Setting double-click-time to nil disables multi-click detection entirely. Setting it to t removes the time limit; Emacs then detects multi-clicks by position only.