feat(api): allow open_win/win_set_buf in the cmdwin in some cases

Problem: As discussed on Matrix, there was some interest in having
`nvim_open_win` again be able to open floats in the cmdwin (e.g: displaying a
hover doc related to what's in the cmdwin). After #23228, this was disallowed.

Solution: Allow `nvim_open_win` in the cmdwin as long as `!enter` and
`buffer != curbuf` (the former can cause all sorts of issues, and the latter
can crash Nvim after closing cmdwin). Also allow `nvim_win_set_buf` in a similar
fashion.

Note that we're not *entirely* sure if this is 100% safe (cmdwin is a
global-state-using-main-loop-calling beast), but this seems to work OK..?

Also:
  - Check the buffer argument of `nvim_open_win` earlier, and abort if it's
    invalid (it used to still open a window in this case).

  - Untranslate `e_cmdwin` errors in the API (other errors in the API are not
    translated: although not detailed in the API contract yet, errors are
    supposed to be stable).

1 files changed, 4 insertions, 10 deletions

diff --git a/src/nvim/window.c b/src/nvim/window.c
index e230bde95c..fa7ca53b08 100644
--- a/src/nvim/window.c
+++ b/src/nvim/window.c
@@ -698,15 +698,9 @@ static void cmd_with_count(char *cmd, char *bufp, size_t bufsize, int64_t Prenum
   }
 }
 
-void win_set_buf(Window window, Buffer buffer, bool noautocmd, Error *err)
+void win_set_buf(win_T *win, buf_T *buf, bool noautocmd, Error *err)
+  FUNC_ATTR_NONNULL_ALL
 {
-  win_T *win = find_window_by_handle(window, err);
-  buf_T *buf = find_buffer_by_handle(buffer, err);
-
-  if (!win || !buf) {
-    return;
-  }
-
   tabpage_T *tab = win_find_tabpage(win);
 
   // no redrawing and don't set the window title
@@ -720,7 +714,7 @@ void win_set_buf(Window window, Buffer buffer, bool noautocmd, Error *err)
     api_set_error(err,
                   kErrorTypeException,
                   "Failed to switch to window %d",
-                  window);
+                  win->handle);
   }
 
   try_start();
@@ -729,7 +723,7 @@ void win_set_buf(Window window, Buffer buffer, bool noautocmd, Error *err)
     api_set_error(err,
                   kErrorTypeException,
                   "Failed to set buffer %d",
-                  buffer);
+                  buf->handle);
   }
 
   // If window is not current, state logic will not validate its cursor.