How does vim catch mouse scroll events?

Question

I am writing a text editor.

With 1049 (alternate screen) mode + 1000 (mouse tracking) mode enabled, the terminal is able to catch both mouse scroll events and click events. I do not need the click events.

This has the added disadvantage of not allowing other useful motions with your mouse (example: an external copy paste by highlighting text on the terminal screen).

I say vim, because it has ideal mouse behavior, where with a scroll event the cursor position changes to reflect it, and click events seem not to affect it. In addition, it does so in a manner that does not lead to flickering.

The thing that tells me that it does not use 1049 + 1007 is that something like the external copy-paste (by highlighting text) is possible in vim.

My question: How is vim doing this?

I've scoured the source code and am unable to find it. I've also attempted to trace the escape sequences it writes to the screen. This holds true on 2 terminal emulators I've tried.

Answer 1

It is hard-coded in the copy of libvterm which vim has used for a couple of years, e.g., in state.c:

  case 1000:
  case 1002:
  case 1003:
    settermprop_int(state, VTERM_PROP_MOUSE,
        !val          ? VTERM_PROP_MOUSE_NONE  :
        (num == 1000) ? VTERM_PROP_MOUSE_CLICK :
        (num == 1002) ? VTERM_PROP_MOUSE_DRAG  :
                        VTERM_PROP_MOUSE_MOVE);
    break;

under the supposition that anything which pretends to be xterm implements the any-event and any-button controls. A lot of vim's term.c attempts to determine which xterm-wannabe may be used, since none of them get it all right, but libvterm ignores the checks that had been done.

(If you want to know how a program does something, starting by reading its source code is the most productive route).