local keymap in mode-line doesn't work in terminal

Question

I'm having trouble with getting local keymap:s to work in the mode-line, but only in terminals (with xterm-mouse-mode enabled).

This applies to both the standard mode-line entries (like mode-line-toggle-read-only) and entries I manually add.

For example, the following work in a GUI Emacs (tested on the NS port on macOS), but not in an Emacs running in a terminal. (When clicking on the PRESSME text in the mode-line, the message I was pressed should appear.):

(defun my-press-me ()
  (interactive)
  (message "I was pressed"))

(push '(:propertize "PRESSME"
                    local-map
                    (keymap
                     (mode-line .
                                (keymap
                                 (down-mouse-1 . my-press-me)))))
      mode-line-format)

Interestingly, when I press C-h k and click in the PRESSME text, it reports my-press-me both in a GUI Emacs and when running in the terminal.

Any thoughts?

Answer 1

IIUC this is a bug in Emacs's C code which was fixed recently in Emacs's master branch (i.e. it will be fixed in Emacs-27, but not Emacs-26).

There might be a way to workaround the bug with some fancy hack in xterm-mouse-mode in Emacs<27, but I'm not sure it's worth the effort to try and come up with it.

See https://debbugs.gnu.org/29104 for details.

Answer 2

I can reproduce this with Emacs 24.4 under Linux. I have a solution, but I don't claim to understand why it works. Although describe-key reads a down-mouse-1 event when you press the button, pressing the button actually triggers the binding for mouse-1. This works both in a terminal (with xterm-mouse-mode) and in an X11 window:

(push '(:propertize "PRESSME"
                    local-map
                    (keymap
                     (mode-line .
                                (keymap
                                 (mouse-1 . my-press-me)))))
      mode-line-format)