bug#38296: Allow Option key to be modifier for non-char key and mouse events

[Mattias Engdegård]

27 nov. 2019 kl. 07.40 skrev Richard Stallman <rms@gnu.org>:

> I'd like to understand what that feature does, but I can't tell from
> that text.  Does it come with any documentation?  If so, coulod you
> please show it?

This text is part of the documentation of the mac-KEY-modifier variables in the 
Mac port:

---
The property list form can include the `:button' property for button
number mapping, which becomes active when the value of
`mac-emulate-three-button-mouse' is nil.  The `:button' property can
be either a positive integer specifying the destination of the primary
button only, or a list (VALUE-FOR-PRIMARY-BUTTON VALUE-FOR-MOUSE-2
VALUE-FOR-MOUSE-3 ...) of positive integers specifying the
destinations of multiple buttons in order.  Note that the secondary
button and the button 3 (usually the wheel button) correspond to
mouse-3 and mouse-2 respectively if the value of
`mac-wheel-button-is-mouse-2' is non-nil (default), and mouse-2 and
mouse-3 respectively otherwise.  If a button is mapped to the same
number as its source, then it behaves as if the button were not mapped
so the `:mouse' property becomes in effect instead.
---

> Also, are you proposing we implement this for ALL platforms (that
> would be clean and simple, in principle)?  Or just SOME platforms?

The variables generalised by the patch are specific to the NS port. Whether 
something like this would be useful for other platforms isn't anything I have 
contemplated, but it is not inconceivable --- it would make keys like AltGr 
free to use as modifiers for mouse gestures and non-character keys. (Like the 
Option key on Macs, the AltGr key is used for entering even plain ASCII 
characters such as $@[]{}|\ on many keyboards, making those keys essential for 
almost any task.)