I would like to create user interface in form of popup menu, popup menu similar to that used in Magit.
Definition of Popup
Popup in context of this question means little temporary window that contains collection of menu items so that user can select one and only one of these items.
Position on the Screen
The popup is allowed to appear in any part of the screen, but it's desirable that it should be quite obvious and thus it should appear beside currently active window.
Contents of Popup Buffer
Items should be displayed in form of pretty table. Pretty is context of
the question means visually appealing, this effect can be most easily
achieved by putting menu items into straight rows, see
complete--insert-string for example. This paragraph serves for additional
clarification, you can do it in your own way, this won't render your answer
Selection of Menu Item
The selection is expected to be performed by a single key-pressing or, optionally with a mouse (although it's not that important so answers containing propositions that don't support mouse are legal). If you propose solution that supports mouse, please note that user should be able to select a menu item in an intuitive way, that is, by left-button clicking on desired choice.
N.B. mouse can be used in many ways and alternative ways to indicate a choice are also welcomed.
Elimination of Popup
Once user has selected a menu item in a way described above, the buffer and thus its window should be eliminated from view as well as killed. The window that has been active before invocation of the popup menu should get focus (that is, become active) again.
Returned Value and Arguments
Preferably, this consequence of actions should result in a Lisp object returned. The Lisp object can be either:
nil— this indicates that user has aborted the popup menu either by pressing C-g or in some other way †.
string— string (it's allowed to use a symbol) should be
string-equalto one of the strings supplied to the popup menu as collection of actual items.
Alternative ways to let the rest of program know user's choice, or, possibly, its absence, are acceptable. However, if it's not clear how else it can be performed I ask all answerers improvise and don't ask me for further clarification of this aspect.
This is all for returned value. As for input parameters, they should at least include collection of strings which represent possible choices (that is, menu items).
Expected answer can be of the following forms:
Sufficient code snippet that allows educated reader to write function like that described above; it's not expected or necessary to write entire working function. However, to avoid uncertainty (can considerable parts of code be omitted?), I should note that missing parts of the snippet should be described in textual component of answer.
A link to existing library that implements similar functionality. To avoid uncertainty, I should note that similar in our case means that the library can be used to create popup (see definition above) that has at least 2 or 3 features described above. If proposed library is different to the point where the previously stated condition cannot be met, each such case will be judged independently and will be always upvoted if OP deems it useful.
Description of built-in Emacs functions or third-party ones that can be used to implement any feature described in section «Features», see above. To avoid uncertainty, please state clearly how your answer can be useful for future readers who want to implement popup, popup menu similar to that used in Magit.
† Alternative ways to abort the popup menu may include the following (but not limited to these):
clicking outside of popup menu window;
killing of buffer containing the popup without making a choice.