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))) {