From 22d829089fc2ca153f1541abe1d5415d311b33d3 Mon Sep 17 00:00:00 2001
From: Leo Butler <leo.butler@umanitoba.ca>
Date: Tue, 12 Mar 2024 15:11:27 -0500
Subject: [PATCH] lisp/ox-beamer.el: constrain use of
 org-beamer-frame-environment

* lisp/ox-beamer.el (org-beamer--format-frame, org-beamer-template):
Only use `org-beamer-frame-environment' when a frame is marked as
fragile and the frame's contents include either \begin{frame} or
\end{frame}.  When `org-beamer-frame-environment' is used and not
equal to "frame", set `org-beamer--frame-environment-used' to t.  When
`org-beamer--frame-environment-used' is t, `org-beamer-template' emits
a definition of the alternative frame environment.

* lisp/ox-beamer.el (org-beamer--frame-environment-used): New
variable.

Refs: https://list.orgmode.org/orgmode/87bk7jeik8.fsf@localhost/
https://list.orgmode.org/87a5nux3zr.fsf@t14.reltub.ca/T/
---
 lisp/ox-beamer.el | 26 ++++++++++++++++++++++----
 1 file changed, 22 insertions(+), 4 deletions(-)

diff --git a/lisp/ox-beamer.el b/lisp/ox-beamer.el
index 4fad37b59..a79490194 100644
--- a/lisp/ox-beamer.el
+++ b/lisp/ox-beamer.el
@@ -167,6 +167,10 @@ used inside beamer slides."
 
 ;;; Internal Variables
 
+(defvar org-beamer--frame-environment-used nil
+  "Nil unless `org-beamer-frame-environment' is used.
+See `org-beamer--frame-environment'.")
+
 (defconst org-beamer-column-widths
   "0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.0 :ETC"
   "The column widths that should be installed as allowed property values.")
@@ -429,8 +433,21 @@ used as a communication channel."
 	  ;; among `org-beamer-verbatim-elements'.
 	  (org-element-map headline org-beamer-verbatim-elements 'identity
 			   info 'first-match))
-         (frame (or (and fragilep org-beamer-frame-environment)
-                    "frame")))
+         ;; If FRAGILEP is non-nil and CONTENTS contains an occurrence
+         ;; of \begin{frame} or \end{frame}, then set the FRAME
+         ;; environment to be `org-beamer-frame-environment';
+         ;; otherwise, use "frame". If the selected environment is not
+         ;; "frame", then set `org-beamer--frame-environment-used' to
+         ;; t.
+         (frame (let ((selection
+                       (or (and fragilep
+                                (or (string-search "\\begin{frame}" contents)
+                                    (string-search "\\end{frame}" contents))
+                                org-beamer-frame-environment)
+                           "frame")))
+                  (unless (string= selection "frame")
+                    (setq org-beamer--frame-environment-used t))
+                  selection)))
     (concat "\\begin{" frame "}"
 	    ;; Overlay specification, if any. When surrounded by
 	    ;; square brackets, consider it as a default
@@ -851,8 +868,9 @@ holding export options."
      (org-latex--insert-compiler info)
      ;; Document class and packages.
      (org-latex-make-preamble info)
-     ;; Define the alternative frame environment.
-     (unless (equal "frame" org-beamer-frame-environment)
+     ;; Define the alternative frame environment, if needed.
+     (when org-beamer--frame-environment-used
+       (setq org-beamer--frame-environment-used nil)
        (format "\\newenvironment<>{%s}[1][]{\\begin{frame}#2[environment=%1$s,#1]}{\\end{frame}}\n"
                org-beamer-frame-environment))
      ;; Insert themes.
-- 
2.43.0