Child frame mispositioned after parent frame resize until next input

[ArthurHeymans]

Problem
When using Corfu in the minibuffer and resizing the Emacs frame, the completion popup child frame stays at its old screen position instead of following the minibuffer. It only gets repositioned when I type the next character.
This happens because:

1. Corfu repositions its popup via post-command-hook
2. Frame resize doesn't trigger a command, so the hook doesn't run
3. The popup position is only recalculated on the next user input
   This affects all Emacs backends (GTK, PGTK, Lucid, etc.) since it's fundamental to how Emacs events work.

Proposed Solution
I've been investigating this in Emacs core and implemented a new resize-frame event mechanism, similar to the existing move-frame event. This generates an event immediately when a frame is resized, allowing packages to respond without waiting for redisplay.
The implementation adds:

• RESIZE_FRAME_EVENT - new event type
• resize-frame-functions - hook that runs immediately on frame resize (unlike window-size-change-functions which only runs during redisplay)
• handle-resize-frame - event handler in frame.el
  Branch with the Emacs changes: https://github.com/ArthurHeymans/emacs/tree/feature/resize-frame-event

Suggested Corfu Integration
With this Emacs change, Corfu could hook into resize-frame-functions to reposition the popup immediately:
(defun corfu--handle-resize-frame (frame)
"Reposition Corfu popup when FRAME is resized."
(when (and corfu--frame
(frame-live-p corfu--frame)
(eq frame (frame-parent corfu--frame)))
(corfu--exhibit)))
;; In corfu--setup:
(add-hook 'resize-frame-functions #'corfu--handle-resize-frame)
;; In corfu--teardown:
(remove-hook 'resize-frame-functions #'corfu--handle-resize-frame)

Do you think this is the right approach to fix this issue? Any feedback on the approach would be appreciated before I pursue getting this into Emacs upstream.

[minad]

I think it would make sense to simply call corfu-quit for frame resizing. Corfu doesn't have to handle every edge case and it often bails out, e.g., when you switch windows.