What can I do to programmatically add to contextual menus? Is something similar to AddMenuCommands available?

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There's an option ContextMenu, which you seem to be able to set at cell, notebook, or even box level. They all default to something like

FEPrivateFrontEndResource["ContextMenus", "something"]


That info seems to be in the ContextMenus.tr file in the \$InstallationDirectory, SystemFiles, FrontEnd, TextResources.

In any case, all that that option needs is a list of MenuItem[...]. You can look at that file (or at MenuSetup.tr) to get a hang on the syntax. Then it's just setting the option.

Apart from the brute force method of reading the file, creating your own, adding your Menus, and setting that option, I still haven't found a way to "append" the MenuItems. Only to replace... I'll edit if I find something

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You're right...you can't append menu items. But you missed a second source of context menus...the ComponentwiseContextMenu option. ContextMenu is used where you can easily dictate context menus by style, whereas ComponentwiseContextMenu` allows you to determine context menus which belong to certain box types, the various cell-related widgets, etc., which may be difficult or impossible to apply styles to. –  John Fultz Mar 6 '12 at 15:52