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I'd like to enable changing the font size by holding down the right mouse button and using the scroll-wheel. Emacs does however treat mouse buttons as keys, so this obviously cannot be achieved directly by a straight-forward (define-key ... (kbd ...) ...) binding.

Since I also don't want to disable the standard actions performed by "click and release" of the right mouse button, I can't just bind something to <up/down-mouse-3>. Instead, I'd like to bind a command to the scroll events <mouse-4> and <mouse-5>, that detects, whether the right mouse button is being held.

Is this possible?

Related questions point to the variable track-mouse, but it doesn't seem, like that method can distinguish different mouse buttons.

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  • You might be able to use something like the answers to this question to detect if the last pressed key was <down-mouse>, then set a variable that you can check. Something based on key-chord.el might also be useful, but that would probably require more work to figure out. An easier solution, might be to add something to the right-click context menu (like a minor mode that binds scrolling to resize).
    – jirassimok
    Apr 8, 2020 at 22:30

2 Answers 2

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Library zoom-frm.el lets you do either of these things using the mouse wheel:

  • Zoom the buffer text size, for all windows showing a given buffer. This is called text-scaling.
  • Zoom the frame text size, for a given frame or for all frames.

It, in turn, requires these libraries:

See also Changing Font Size.

These are bindings recommended in zoom-frm.el for the mouse wheel:

(global-set-key (vector (list 'control mouse-wheel-down-event))
                'zoom-in)
(global-set-key (vector (list 'control 'meta mouse-wheel-down-event))
                'zoom-all-frames-in)
(global-set-key (vector (list 'control  mouse-wheel-up-event))
                'zoom-out)
(global-set-key (vector (list 'control 'meta mouse-wheel-up-event))
                'zoom-all-frames-out)

Perhaps you can provide similar bindings for simultaneous use of the wheel and holding mouse-3 (right button) pressed. I don't know, and I'm not really motivated to find out. Perhaps someone else here will help with that.

For buffer (not frame) zooming, you might also want to use library face-remap+.el. It lets buffer resizing also zoom the window size accordingly (horizontally, vertically, or both). That way, you can take advantage of the space freed up by resizing (text-scaling) to a smaller font.

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  • Binding the mouse-wheel is really white easy; I have my own setup for that. The problem is mostly avoiding the use of they keyboard while scrolling through other peoples source code.
    – kdb
    Apr 8, 2020 at 20:12
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mapping behavior similar to browser might be helpful,

  • zoom in : Ctrl + mouse-scroll-up and Ctrl + Shift + =
  • zoom out : Ctrl + mouse-scroll-down and Ctrl + -
  • zoom 1x : Ctrl + 0

these config are for the single buffer

(global-set-key [C-mouse-4] 'text-scale-increase)
(global-set-key [(control ?+)] 'text-scale-increase)
(global-set-key [C-mouse-5] 'text-scale-decrease)
(global-set-key [(control ?-)] 'text-scale-decrease)
(global-set-key (kbd "C-0") '(lambda () (interactive)
                               (text-scale-adjust
                                (- text-scale-mode-amount))
                               (text-scale-mode -1)))
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