CPack/WiX: Restore default (missing) InstallScope from 3.28 and lower

In commit 60661f6 (CPack/WiX: Make InstallScope configurable,
2023-11-07, v3.29.0-rc1~413^2) the `CPACK_WIX_INSTALL_SCOPE` default was
set to `perMachine`.  While installers created with `perMachine` have
better behavior on their own than installers created without any
`InstallScope`, they do not cleanly convert existing installations on
updates.

Fixes: #26029

Author: bradking

Help/cpack_gen/wix.rst

@@ -346,7 +346,7 @@ Windows using WiX.
  This variable can be optionally set to specify the ``InstallScope``
  of the installer:
 
- ``perMachine`` (default)
+ ``perMachine``
    Create an installer that installs for all users and requires
    administrative privileges.  Start menu entries created by the
    installer are visible to all users.
@@ -357,6 +357,8 @@ Windows using WiX.
  ``NONE``
    Create an installer without any ``InstallScope`` attribute.
 
+   This is the default to preserve compatibility with CPack 3.28 and older.
+
    .. deprecated:: 3.29
 
      This value is only for compatibility with the inconsistent behavior used
@@ -365,4 +367,13 @@ Windows using WiX.
      but the start menu entry and uninstaller registration are created only
      for the current user.
 
+   .. warning::
+
+     An installation performed by an installer created without any
+     ``InstallScope`` cannot be cleanly updated or replaced by an
+     installer with an ``InstallScope``.  In order to transition
+     a project's installers from ``NONE`` to ``perMachine``, the
+     latter installer should be distributed with instructions to
+     first manually uninstall any older version.
+
  See https://wixtoolset.org/docs/v3/xsd/wix/package/

Help/release/3.29.rst

@@ -193,13 +193,6 @@ Other Changes
 * The :variable:`CPACK_PRODUCTBUILD_DOMAINS` variable now defaults to true.
   See policy :policy:`CMP0161`.
 
-* The :cpack_gen:`CPack WIX Generator` now produces WiX MSI installers
-  that create start menu and uninstall entries for all users by default,
-  as documented by the :variable:`CPACK_WIX_INSTALL_SCOPE` variable
-  ``perMachine`` value.  Previously, without a custom WiX template,
-  it produced installers that would only create start menu and uninstall
-  entries for the current user, even though they install for all users.
-
 Updates
 =======
 
@@ -218,3 +211,22 @@ Changes made since CMake 3.29.0 include the following.
 * These versions made no changes to documented features or interfaces.
   Some implementation updates were made to support ecosystem changes
   and/or fix regressions.
+
+3.29.5
+------
+
+* The :cpack_gen:`CPack WIX Generator`'s :variable:`CPACK_WIX_INSTALL_SCOPE`
+  variable, new in 3.29, now defaults to ``NONE``.  This restores
+  compatibility with behavior of 3.28 and below: without a custom WiX
+  template, it produces installers that only create start menu and
+  uninstall entries for the current user, even though they install
+  for all users.
+
+  In 3.29.0 through 3.29.4, ``CPACK_WIX_INSTALL_SCOPE`` defaulted to
+  ``perMachine``.  This created MSI installers that create start menu
+  and uninstall entries for all users by default.  While this behavior
+  is better on its own, these installers do not cleanly replace existing
+  installations created with MSI installers produced by 3.28 and below.
+  3.29.5 reverts the default for compatibility.  Projects may transition
+  to ``perMachine`` on their own schedule by setting
+  ``CPACK_WIX_INSTALL_SCOPE``.

Modules/Internal/CPack/CPackWIX.cmake

@@ -20,5 +20,5 @@ if(NOT CPACK_WIX_LIGHT_EXECUTABLE)
 endif()
 
 if(NOT DEFINED CPACK_WIX_INSTALL_SCOPE)
-  set(CPACK_WIX_INSTALL_SCOPE "perMachine")
+  set(CPACK_WIX_INSTALL_SCOPE "NONE")
 endif()