The low-level category consists of those events triggered by mouse button motion in a single direction, such as down or up. The high-level category consists of those events triggered by more complex mouse button motion, such as click or double-click. For ABL, corresponding low-level and high-level mouse events are analogous to equivalent key label and key function events. Like key label and key function events, or three-button and portable mouse events, low-level mouse events take priority over corresponding high-level mouse events. For example, if you define a MOUSE–SELECT–UP and MOUSE–SELECT–CLICK trigger on the same widget, only the MOUSE–SELECT–UP trigger executes.
Like portable events, use high-level mouse events exclusively whenever possible. If you must use low-level events, do not mix low-level and high-level event triggers on the same widget. The processing of both classes of events on the same widget can lead to unintended results.
While the AVM always recognizes corresponding low-level and high-level events, it executes only one trigger for both. (The same is true for corresponding key label and key function events, as well as portable and three-button mouse events). The AVM looks first for a trigger on a low-level event. If there is no trigger on a low-level event, the AVM looks for a trigger on a high-level event. Once it finds and fires a trigger, it waits for the next event. However, each graphic interface may recognize and pass high-level events to the AVM after recognizing a different combination of low-level events. For example, one system might recognize a double-click on a down and another system might recognize the double-click on an up mouse event. This causes mixed event sequences to differ significantly from interface to interface.
Thus, much like using pixels for frame and window layout, the mixing of low- and high-level mouse events creates portability problems between graphic interfaces. Furthermore, there is no guarantee that the expected event sequences in the same graphic interface might not change in future versions of that interface.
