transient--describe-function: handle describe-function overrides

[benthamite]

Problem

transient--describe-function captures the help buffer via temp-buffer-window-setup-hook. This works with the built-in describe-function, which displays its output via with-help-window and therefore triggers the hook.

However, a common pattern in the Emacs community is to override describe-function with packages like helpful:

(advice-add 'describe-function :override #'helpful-function)

This is recommended by Doom Emacs, Spacemacs, and countless dotfiles. helpful-function creates and displays its buffer via pop-to-buffer rather than with-help-window, so temp-buffer-window-setup-hook never fires, buffer stays nil, and (set-buffer nil) signals (wrong-type-argument stringp nil).

This means pressing ? followed by a suffix key in any transient menu errors out for anyone using this pattern.

Fix

     (describe-function fn)
-    (set-buffer buffer))
+    (when buffer (set-buffer buffer)))

When the hook fires (standard describe-function), behavior is unchanged. When it doesn't (helpful or any other override that bypasses with-help-window), we skip the set-buffer call entirely — the override's pop-to-buffer already selected the correct window and set the current buffer, so no further action is needed.

Context

This is in the spirit of the robustness improvements in #208 and #213, which led to commit 732ec97 rewriting transient--describe-function because it was "too fragile" — it assumed (help-buffer) would return the correct buffer, which broke when packages like epithet renamed help buffers. The current implementation replaced that assumption with temp-buffer-window-setup-hook, but this hook has the same fragility when describe-function itself is replaced with something that doesn't use with-help-window.

[tarsius]

Do we even have to call set-buffer in that case, or would (when buffer (set-buffer buffer)) be more appropriate?

[benthamite]

Good point — updated. When the hook doesn't fire, the override's pop-to-buffer has already selected the window and set the current buffer, so there's nothing for set-buffer to do.