fix(cwd): stale buffer names after temp context-switch #41433

Problem:
1. `ctx_dirs_restore()` is the only chdir site that doesn't re-shorten
   buffer names; `post_chdir()`, `update_cwd()` and `do_autochdir()` all
   call `shorten_fnames(true)`.  So after a temp window-context switch
   that moved the CWD, every buffer's `b_fname` is still relative to the
   other directory.  ui2 renders messages in a float, and entering it is
   such a switch, so with a `:bcd` in a `BufReadPost` handler 'statusline'
   "%f" shows ".config/nvim/init.lua" while the CWD is already
   `~/.config/nvim`, and :write resolves the name against it:
   ```
   E212: Can't open file for writing: no such file or directory
   ```
2. `msg_multihl()` leaves `msg_ext_id` pointing at the caller's storage
   when nothing was emitted (e.g. 'msg_silent'): the reset only ran on
   a flush that produced chunks.  The next message then ships a dead
   stack frame as its msg_show id.  After ":silent write" the id is
   buf_write()'s `msg_id[MAXPATHL + 32]`:
   ```
   id = "\0\0\0\0\0\0\0\0\29\0(<C6>k\24R\17p<C7><C7><C7>\1\0\0\0..."
   ```

Solution:
1. `shorten_fnames(true)` after restoring the CWD.  Drop `cs_save_sfname`,
   which was a partial workaround for the same bug.
2. Release the id in `msg_multihl()`, where it is set and the caller's
   frame is still alive. Stop reading `.data.integer` out of a String
   union member.

0 files changed, 0 insertions, 0 deletions