Made prompt_toolkit's mouse_support optional

Introducing a new environment variable MOUSE_SUPPORT for this,
which can either be True or False. Default is False.

docs/envvars.rst

@@ -146,6 +146,13 @@ applicable.
     * - XONSH_INTERACTIVE
       - 
       - ``True`` if xonsh is running interactively, and ``False`` otherwise.
+    * - MOUSE_SUPPORT
+      - ``False``
+      - Enable mouse support in the prompt_toolkit shell. This allows clicking
+        for positioning the cursor or selecting a completion. In some terminals
+        however, this disables the ability to scroll back through the history
+        of the terminal.
+        (Only usable with SHELL_TYPE=prompt_toolkit)
     * - XONSH_SHOW_TRACEBACK
       - ``False`` but not set
       - Controls if a traceback is shown exceptions occur in the shell. Set ``True`` 

xonsh/environ.py

@@ -62,6 +62,7 @@ DEFAULT_ENSURERS = {
     'CASE_SENSITIVE_COMPLETIONS': (is_bool, to_bool, bool_to_str),
     'BASH_COMPLETIONS': (is_env_path, str_to_env_path, env_path_to_str),
     'TEEPTY_PIPE_DELAY': (is_float, float, str),
+    'MOUSE_SUPPORT': (is_bool, to_bool, bool_to_str),
 }
 
 #
@@ -581,6 +582,7 @@ BASE_ENV = {
     'LC_MONETARY': locale.setlocale(locale.LC_MONETARY),
     'LC_NUMERIC': locale.setlocale(locale.LC_NUMERIC),
     'XONSH_VERSION': XONSH_VERSION,
+    'MOUSE_SUPPORT': False,
 }
 
 try:

xonsh/prompt_toolkit_shell.py

@@ -59,11 +59,12 @@ class PromptToolkitShell(BaseShell):
         """Enters a loop that reads and execute input from user."""
         if intro:
             print(intro)
+        mouse_support = builtins.__xonsh_env__.get('MOUSE_SUPPORT')
         while not builtins.__xonsh_exit__:
             try:
                 token_func, style_cls = self._get_prompt_tokens_and_style()
                 line = get_input(
-                    mouse_support=True,
+                    mouse_support=mouse_support,
                     auto_suggest=AutoSuggestFromHistory(),
                     get_prompt_tokens=token_func,
                     style=style_cls,