Enable xterm-mouse-mode by default in xterm

* lisp/xt-mouse.el (xterm-mouse-mode): Change default value of
xterm-mouse-mode to t.
* lisp/term/xterm.el (xterm--init): Enable xterm-mouse-mode if
the default value is still set.
* etc/NEWS: Document new behavior.

etc/NEWS

@@ -41,6 +41,12 @@ why the mark trace buffer is enabled by default.
 
 * Startup Changes in Emacs 31.1
 
+** When run inside xterm, 'xterm-mouse-mode' is turned on by default.
+This means that the mouse will work by default inside xterm terminals.
+If your terminal does not behave properly with xterm mouse tracking
+enabled, you can disable mouse tracking by putting '(xterm-mouse-mode
+-1)' in your init file.
+
 
 * Changes in Emacs 31.1
 

lisp/term/xterm.el

@@ -907,6 +907,8 @@ We run the first FUNCTION whose STRING matches the input events."
 
   (when xterm-set-window-title
     (xterm--init-frame-title))
+  (when xterm-mouse-mode
+    (xterm-mouse-mode 1))
   ;; Unconditionally enable bracketed paste mode: terminals that don't
   ;; support it just ignore the sequence.
   (xterm--init-bracketed-paste-mode)

lisp/xt-mouse.el

@@ -358,6 +358,8 @@ single clicks are supported.  When turned on, the normal xterm
 mouse functionality for such clicks is still available by holding
 down the SHIFT key while pressing the mouse button."
   :global t :group 'mouse
+  :init-value t
+  :version "31.1"
   (funcall (if xterm-mouse-mode 'add-hook 'remove-hook)
            'terminal-init-xterm-hook
            'turn-on-xterm-mouse-tracking-on-terminal)