fish_vi_cursor: add new variable for external cursor mode (#9565)

* add new variable for external cursor mode

* fix backwards compatibility

* add documentation

* document change in changelog

(cherry picked from commit f5506803d7)

CHANGELOG.rst

@@ -47,6 +47,7 @@ Interactive improvements
 - The ``repaint-mode`` binding will now only move the cursor if there is repainting to be done. This fixes alt+something bindings in vi-mode (:issue:`7910`).
 - A new ``clear-screen`` bind function is used for :kbd:`Alt`\ +\ :kbd:`l` by default. This clears the screen and repaints the existing prompt at first,
   so it eliminates visible flicker unless the terminal is very slow (:issue:`10044`).
+- A new variable, :envvar:`fish_cursor_external`, can be used to specify to cursor shape when a command is launched. When unspecified, the value defaults to the value of :envvar:`fish_cursor_default` (:issue:`4656`).
 
 Improved prompts
 ^^^^^^^^^^^^^^^^

doc_src/interactive.rst

@@ -421,6 +421,9 @@ The ``fish_vi_cursor`` function will be used to change the cursor's shape depend
    # Set the replace mode cursors to an underscore
    set fish_cursor_replace_one underscore
    set fish_cursor_replace underscore
+   # Set the external cursor to a line. The external cursor appears when a command is started. 
+   # The cursor shape takes the value of fish_cursor_default when fish_cursor_external is not specified.
+   set fish_cursor_external line
    # The following variable can be used to configure cursor shape in
    # visual mode, but due to fish_cursor_default, is redundant here
    set fish_cursor_visual block

share/functions/fish_vi_cursor.fish

@@ -92,7 +92,10 @@ function fish_vi_cursor -d 'Set cursor shape for different vi modes'
 
     echo "
           function fish_vi_cursor_handle_preexec --on-event fish_preexec
-              set -l varname fish_cursor_default
+              set -l varname fish_cursor_external
+              if not set -q \$varname
+                set varname fish_cursor_default
+              end
               if not set -q \$varname
                 set varname fish_cursor_unknown
               end