wezterm/docs/config/lua/keyassignment/ExtendSelectionToMouseCursor.md

ExtendSelectionToMouseCursor

Extends the current text selection to the current mouse cursor position. The mode argument can be one of Cell, Word or Line to control the scope of the selection.

It is also possible to leave the mode unspecified like this:

return {
  mouse_bindings = {
    {
      event={Up={streak=1, button="Left"}},
      mods="SHIFT",
      -- Note that there is no `wezterm.action` here
      action={ExtendSelectionToMouseCursor={}},
    },
  }
}

when unspecified, wezterm will use a default mode which at the time of writing is Cell, but in a future release may be context sensitive based on recent actions.