My application is context-sensisitve and I dynamically build menus for the main window / context/popup, and other places. I typically know if a given menu command will be valid given the current state of the application. Is it better practice to DISABLE/GREY the menu options which currently do not apply OR since I'm generating the menu anyway, OMIT them entirely?
The application is a Java/Swing is anyone is curious. The question seems GUI toolkit agnostic but may be platform dependent.
The old apple guidelines say to Disable for fixed menus (in the menu bar), and omit for context menus.
I guess the motivation is that a context menu is supposed to only show options that are available to the particular context, and the main menus are supposed to show all commands, so the user knows where "Save" would be even if it's not selectable at the moment.
For right-click menus, I'd say that if the item is applicable to what was right-clicked but is for some other outside reason unavailable, disable it. If it is not applicable to the right-clicked thing then hide it as there's no chance of it ever showing up. Case in point:
When I right-click on the background area of this page in Firefox the first four items are Back, Forward, Reload, and Stop. Forward and Stop are disabled because they aren't valid actions right now (I have no forward history and the page is not loading anymore). These four guys are very consistently offered, they are expected, global, often-used commands. They are the four main "navigation" controls and by default they have toolbar counterparts (in the form of big dedicated buttons).
However, if I right-click on an image, I get completely different options in the context-menu all related to viewing, saving, and copying the image under where I clicked. These options don't appear at all (not even disabled) under normal use because they are very specific to what I right-clicked on. When right-clicking on the background area, Stop and Forward, while currently not valid actions, are still applicable to what I clicked on (the page) but they are unavailable for other reasons...
Like the rule for menus on the top menu bar, the goal is not to surprise users with commands suddenly appearing for, from the user's point of view, inexplicable reasons.