diff --git a/ci/build.ps1 b/ci/build.ps1
index 627dc69e88..a7a456f67a 100644
--- a/ci/build.ps1
+++ b/ci/build.ps1
@@ -173,6 +173,9 @@ if (-not $NoTests) {
   # Functional tests
   # The $LastExitCode from MSBuild can't be trusted
   $failed = $false
+
+  # Run only this test file:
+  # $env:TEST_FILE = "test\functional\foo.lua"
   cmake --build . --config $cmakeBuildType --target functionaltest -- $cmakeGeneratorArgs 2>&1 |
     foreach { $failed = $failed -or
       $_ -match 'functional tests failed with error'; $_ }
diff --git a/runtime/doc/usr_21.txt b/runtime/doc/usr_21.txt
index 99a78e7b05..44d653a1a7 100644
--- a/runtime/doc/usr_21.txt
+++ b/runtime/doc/usr_21.txt
@@ -339,21 +339,6 @@ window, move the cursor to the filename and press "O".  Double clicking with
 the mouse will also do this.
 
 
-UNIX AND MS-WINDOWS
-
-Some people have to do work on MS-Windows systems one day and on Unix another
-day.  If you are one of them, consider adding "slash" and "unix" to
-'sessionoptions'.  The session files will then be written in a format that can
-be used on both systems.  This is the command to put in your |init.vim| file:
->
-	:set sessionoptions+=unix,slash
-
-Vim will use the Unix format then, because the MS-Windows Vim can read and
-write Unix files, but Unix Vim can't read MS-Windows format session files.
-Similarly, MS-Windows Vim understands file names with / to separate names, but
-Unix Vim doesn't understand \.
-
-
 SESSIONS AND SHADA
 
 Sessions store many things, but not the position of marks, contents of
diff --git a/src/nvim/ex_docmd.c b/src/nvim/ex_docmd.c
index 79cf1794b1..d87dd29f88 100644
--- a/src/nvim/ex_docmd.c
+++ b/src/nvim/ex_docmd.c
@@ -8089,6 +8089,10 @@ static void close_redir(void)
   do { if (FAIL == put_line(fd, (s))) { return FAIL; } } while (0)
 
 /// ":mkexrc", ":mkvimrc", ":mkview", ":mksession".
+///
+/// Legacy 'sessionoptions' flags SSOP_UNIX, SSOP_SLASH are always enabled.
+///   - SSOP_UNIX: line-endings are always LF
+///   - SSOP_SLASH: filenames are always written with "/" slash
 static void ex_mkrc(exarg_T *eap)
 {
   FILE        *fd;
@@ -9112,6 +9116,8 @@ char_u *expand_sfile(char_u *arg)
 
 /// Writes commands for restoring the current buffers, for :mksession.
 ///
+/// Legacy 'sessionoptions' flags SSOP_UNIX, SSOP_SLASH are always enabled.
+///
 /// @param dirnow  Current directory name
 /// @param fd  File descriptor to write to
 ///
@@ -9840,12 +9846,10 @@ static char *ses_escape_fname(char *name, unsigned *flagp)
   char *p;
   char *sname = (char *)home_replace_save(NULL, (char_u *)name);
 
-  if (*flagp & SSOP_SLASH) {
-    // change all backslashes to forward slashes
-    for (p = sname; *p != NUL; MB_PTR_ADV(p)) {
-      if (*p == '\\') {
-        *p = '/';
-      }
+  // Always SSOP_SLASH: change all backslashes to forward slashes.
+  for (p = sname; *p != NUL; MB_PTR_ADV(p)) {
+    if (*p == '\\') {
+      *p = '/';
     }
   }
 
diff --git a/src/nvim/option_defs.h b/src/nvim/option_defs.h
index 227c0fec4d..c5d8b134c4 100644
--- a/src/nvim/option_defs.h
+++ b/src/nvim/option_defs.h
@@ -577,8 +577,8 @@ static char *(p_ssop_values[]) = {
 # define SSOP_HELP              0x040
 # define SSOP_BLANK             0x080
 # define SSOP_GLOBALS           0x100
-# define SSOP_SLASH             0x200
-# define SSOP_UNIX              0x400  // Deprecated, not used.
+# define SSOP_SLASH             0x200  // Deprecated, always set.
+# define SSOP_UNIX              0x400  // Deprecated, always set.
 # define SSOP_SESDIR            0x800
 # define SSOP_CURDIR            0x1000
 # define SSOP_FOLDS             0x2000
diff --git a/test/functional/ex_cmds/mksession_spec.lua b/test/functional/ex_cmds/mksession_spec.lua
index 305850a09e..949724bb53 100644
--- a/test/functional/ex_cmds/mksession_spec.lua
+++ b/test/functional/ex_cmds/mksession_spec.lua
@@ -96,7 +96,8 @@ describe(':mksession', function()
 
   it('restores CWD for :terminal buffers #11288', function()
     local cwd_dir = funcs.fnamemodify('.', ':p:~'):gsub([[[\/]*$]], '')
-    local session_path = cwd_dir..get_pathsep()..session_file
+    cwd_dir = cwd_dir:gsub([[\]], '/')  -- :mksession always uses unix slashes.
+    local session_path = cwd_dir..'/'..session_file
 
     command('cd '..tab_dir)
     command('terminal echo $PWD')
@@ -108,7 +109,7 @@ describe(':mksession', function()
     clear()
     command('silent source '..session_path)
 
-    local expected_cwd = cwd_dir..get_pathsep()..tab_dir
+    local expected_cwd = cwd_dir..'/'..tab_dir
     matches('^term://'..pesc(expected_cwd)..'//%d+:', funcs.expand('%'))
     command('qall!')
   end)
diff --git a/test/functional/helpers.lua b/test/functional/helpers.lua
index 3ffc6137d6..53c4d140b6 100644
--- a/test/functional/helpers.lua
+++ b/test/functional/helpers.lua
@@ -794,7 +794,7 @@ function module.alter_slashes(obj)
     end
     return ret
   else
-    assert(false, 'Could only alter slashes for tables of strings and strings')
+    assert(false, 'expected string or table of strings, got '..type(obj))
   end
 end