Strictly speaking UI and components, the screenshots you attached are not comparable. Let me explain:
The menu component, be it on Linux, Mac, Windows (or any other operating system) is usually made up from two sub components: the menu bar and menu popup. (menu bar being the root container and the menu popup being the sub item / context menu container).
Not a single menu bar has expand handlers. And from usability point they don't need to, as all menu bar items expand, otherwise they wouldn't be there.
Menu popup items DO have expand arrows, but that's another story.
Now let's go over the screenshots you have sent us:
* context menu: as explained above, menu popup items (context menu items included) have arrows;
* outlook: this one uses ribbon interface, which could be regarded as a smart toolbar; toolbars in terms are made of buttons (among other things) and buttons can be push buttons, menu buttons, split buttons, with the latter two having a drop down arrow to indicate that there is and additional action associated with them.
Having the expand arrows in the Metro skin in the first place was a wrong implementation of the design guidelines. We have corrected that [by removing the arrow].
Having said all of the above, it doesn't mean that there won't be expand arrows and period. Instead it means that expand arrows will be added when we have agreed on the looks and implementation.
The UI concepts are evolving. Especially with the rise of mobile devices usage traditional UI paradigms have to be rethought to be compatible and working properly. One such paradigm is the menu, items and sub items.
Examine the attached screenshot. It illustrates properly the concept of having both own action associated with the menu item and sub items, which can be expanded on click -- the separate expand handler.
In MS spec terms, that is called MenuSplitItem i.e. a menu items that has children. Implementing such behaviour in the RadMenu will be absolute breaking change and thus we'll not go on that path.
We will, however, refine our implementation to allow expand handlers.
the Telerik team
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