The type of new imip bar buttons, introduced with bug 990009, is adjusted automatically based on availability of menuitems and can be a simple button, a "menu-button" type button providing additionally options to the user or a "menu" button, only presenting more options on click. We should add a mozmill test to ensure this is always working.

This can probably be based on the work for bug 1010294.

Patch for bug 1010294 also asserts for the existence of Accept and Details buttons. I think the patch for this bug will be an extension of it with asserting Decline button included in it. Is it or we need something more than that here?

There are some more buttons that may appear in different situations. The buttons are basically defined in http://mxr.mozilla.org/comm-central/source/calendar/lightning/content/imip-bar-overlay.xul (here you see all possible buttons including the possible menuitems of dropdown buttons), while the decision which button to show is made at http://mxr.mozilla.org/comm-central/source/calendar/base/modules/calItipUtils.jsm#233. On top of this buttons may be modified dynamically in their appearance (e.g. from a drop down button as defined in xul to a simple button, if there's only one menuitem) at http://mxr.mozilla.org/comm-central/source/calendar/lightning/content/imip-bar.js#152. Basically, you can distinct two scenarios: showing only one button or showing multiple. The first applies for attendees on receiving a minor invitation update (->Update), a request to re-confirm the current partstat (->Re-Confirm), an event cancellation (->Delete), a published event (->Add). The Update button also appears for organizers receiving a reply from an attendee. This buttons are always simple buttons. The second scenario applies always when receiving an invitation or major invitation update. In this case, 3 buttons are shown: Accept, Decline and More. The first two buttons may be different ones depending on whether the invitation is a single or recurring event. The Accept button has also a menuitem for tentative acceptance (this was be separate button before). Last but not least, the More button contains any anay other actions (currently only the save-as-copy option). This bug is mainly for the 3-button scenario, but the single buttons may deserve a test as well. If you have further questions, let me know.