fix(mksession): don't store floats in session #18635

Problem:
If there are floating windows when `:mksession` runs, the session cannot
be properly restored.

Solution:
Change `:mksession` to skip floating windows. This matches Vim's
treatment of popup windows.

An alternative approach could have `:mksession` save floating windows
that can be _properly_ restored (rather than skip them entirely, which
is what this PR does). While that would seemingly be a more complete
fix, that could present additional issues since floating windows are
ordinarily created by plugins, and they may no longer be properly under
a plugin's control when restored.

closes #18432

src/nvim/ex_session.c

@@ -181,6 +181,10 @@ static bool ses_do_frame(const frame_T *fr)
 /// @return  non-zero if window "wp" is to be stored in the Session.
 static int ses_do_win(win_T *wp)
 {
+  // Skip floating windows to avoid issues when restoring the Session. #18432
+  if (wp->w_floating) {
+    return false;
+  }
   if (wp->w_buffer->b_fname == NULL
       // When 'buftype' is "nofile" can't restore the window contents.
       || (!wp->w_buffer->terminal && bt_nofile(wp->w_buffer))) {

test/functional/ex_cmds/mksession_spec.lua

@@ -13,6 +13,7 @@ local matches = helpers.matches
 local pesc = helpers.pesc
 local rmdir = helpers.rmdir
 local sleep = helpers.sleep
+local meths = helpers.meths
 
 local file_prefix = 'Xtest-functional-ex_cmds-mksession_spec'
 
@@ -166,4 +167,37 @@ describe(':mksession', function()
     -- Verify that the terminal's working directory is "/".
     screen:expect(expected_screen)
   end)
+
+  it('restores a session when there is a float #18432', function()
+    local tmpfile = file_prefix .. '-tmpfile-float'
+
+    command('edit ' .. tmpfile)
+    local buf = meths.create_buf(false, true)
+    local config = {
+      relative = 'editor',
+      focusable = false,
+      width = 10,
+      height = 3,
+      row = 0,
+      col = 1,
+      style = 'minimal'
+    }
+    meths.open_win(buf, false, config)
+    local cmdheight = meths.get_option('cmdheight')
+    command('mksession ' .. session_file)
+
+    -- Create a new test instance of Nvim.
+    clear()
+
+    command('source ' .. session_file)
+
+    eq(tmpfile, funcs.expand('%'))
+    -- Check that there is only a single window, which indicates the floating
+    -- window was not restored.
+    eq(1, funcs.winnr('$'))
+    -- The command-line height should remain the same as it was.
+    eq(cmdheight, meths.get_option('cmdheight'))
+
+    os.remove(tmpfile)
+  end)
 end)