fix(custom-comments): heavy-box interiors are valid comments

The heavy-box empty and text lines began with a bare decoration char, so in line-comment languages (elisp, Python) C-; C h injected syntax-breaking lines. Prefix the interiors with the comment char and suffix them like cj/--comment-box does. Add the missing min-length guard so small or negative widths error cleanly instead of failing inside make-string. Updated the two characterization assertions to the corrected output.

3 files changed, 54 insertions, 42 deletions

diff --git a/modules/custom-comments.el b/modules/custom-comments.el
index b6919d651..cae911061 100644
--- a/modules/custom-comments.el
+++ b/modules/custom-comments.el
@@ -409,47 +409,57 @@ LENGTH is the total width of each line."
   (let* ((current-column-pos (current-column))
          (comment-char (if (equal cmt-start ";") ";;" cmt-start))
          (comment-end-char (if (string-empty-p cmt-end) comment-char cmt-end))
-         (available-width (- length current-column-pos
-                            (length comment-char)
-                            (length comment-end-char)
-                            2))  ; spaces around content
-         (border-line (make-string available-width (string-to-char decoration-char)))
-         (text-length (length text))
-         (padding-each-side (max 1 (/ (- available-width text-length) 2)))
-         (right-padding (if (= (% (- available-width text-length) 2) 0)
-                           padding-each-side
-                         (1+ padding-each-side))))
-    ;; Top border
-    (insert comment-char " " border-line " " comment-end-char)
-    (newline)
+         (min-length (+ current-column-pos
+                        (length comment-char)
+                        2  ; spaces around content
+                        (length comment-end-char)
+                        6)))  ; 3 border chars + text space + 3 border chars
+    (when (< length min-length)
+      (error "Length %d is too small to generate comment (minimum %d)" length min-length))
+    (let* ((available-width (- length current-column-pos
+                              (length comment-char)
+                              (length comment-end-char)
+                              2))  ; spaces around content
+           (border-line (make-string available-width (string-to-char decoration-char)))
+           (text-available (- available-width 4))  ; 2 side decorations, 2 spaces
+           (text-length (length text))
+           (padding-each-side (max 1 (/ (- text-available text-length) 2)))
+           (right-padding (if (= (% (- text-available text-length) 2) 0)
+                             padding-each-side
+                           (1+ padding-each-side)))
+           ;; Interior side-border lines repeat the comment prefix and suffix so
+           ;; the empty/text rows stay valid comments in line-comment languages
+           ;; (elisp, Python). Previously they began with a bare decoration char.
+           (empty-line (concat comment-char " " decoration-char
+                               (make-string (- available-width 2) ?\s)
+                               decoration-char " " comment-end-char)))
+      ;; Top border
+      (insert comment-char " " border-line " " comment-end-char)
+      (newline)
 
-    ;; Empty line with side borders
-    (dotimes (_ current-column-pos) (insert " "))
-    (insert decoration-char)
-    (dotimes (_ available-width) (insert " "))
-    (insert " " decoration-char)
-    (newline)
+      ;; Empty line with side borders
+      (dotimes (_ current-column-pos) (insert " "))
+      (insert empty-line)
+      (newline)
 
-    ;; Centered text line
-    (dotimes (_ current-column-pos) (insert " "))
-    (insert decoration-char " ")
-    (dotimes (_ padding-each-side) (insert " "))
-    (insert text)
-    (dotimes (_ right-padding) (insert " "))
-    (insert " " decoration-char)
-    (newline)
+      ;; Centered text line
+      (dotimes (_ current-column-pos) (insert " "))
+      (insert comment-char " " decoration-char " ")
+      (dotimes (_ padding-each-side) (insert " "))
+      (insert text)
+      (dotimes (_ right-padding) (insert " "))
+      (insert " " decoration-char " " comment-end-char)
+      (newline)
 
-    ;; Empty line with side borders
-    (dotimes (_ current-column-pos) (insert " "))
-    (insert decoration-char)
-    (dotimes (_ available-width) (insert " "))
-    (insert " " decoration-char)
-    (newline)
+      ;; Empty line with side borders
+      (dotimes (_ current-column-pos) (insert " "))
+      (insert empty-line)
+      (newline)
 
-    ;; Bottom border
-    (dotimes (_ current-column-pos) (insert " "))
-    (insert comment-char " " border-line " " comment-end-char)
-    (newline)))
+      ;; Bottom border
+      (dotimes (_ current-column-pos) (insert " "))
+      (insert comment-char " " border-line " " comment-end-char)
+      (newline))))
 
 (defun cj/comment-heavy-box ()
   "Insert a heavy box comment with blank lines around centered text.
diff --git a/tests/test-custom-comments-comment-heavy-box.el b/tests/test-custom-comments-comment-heavy-box.el
index 94d4aaa5f..8acb9ff0b 100644
--- a/tests/test-custom-comments-comment-heavy-box.el
+++ b/tests/test-custom-comments-comment-heavy-box.el
@@ -64,8 +64,8 @@ Returns the buffer string for assertions."
     (should (string-match-p "^;; \\*" result))
     ;; Middle line should contain centered text
     (should (string-match-p "Section Header" result))
-    ;; Should have side borders
-    (should (string-match-p "^\\*.*\\*$" result))))
+    ;; Interior side-border lines carry the comment prefix/suffix (not a bare *)
+    (should (string-match-p "^;; \\*.*\\* ;;$" result))))
 
 (ert-deftest test-heavy-box-elisp-custom-decoration ()
   "Should use custom decoration character."
@@ -83,8 +83,8 @@ Returns the buffer string for assertions."
   (let ((result (test-heavy-box-at-column 0 ";;" "" "*" "" 70)))
     ;; Should still generate 5 lines
     (should (= 5 (length (split-string result "\n" t))))
-    ;; Middle line should just have side borders and spaces
-    (should (string-match-p "^\\*.*\\*$" result))))
+    ;; Middle line should just have side borders and spaces, comment-prefixed
+    (should (string-match-p "^;; \\*.*\\* ;;$" result))))
 
 (ert-deftest test-heavy-box-elisp-at-column-0 ()
   "Should work at column 0."
diff --git a/todo.org b/todo.org
index ebaaa7e85..de07399f5 100644
--- a/todo.org
+++ b/todo.org
@@ -989,7 +989,9 @@ Add the buffer-local var, set it on each "Run a test..." selection, use it as th
 *** TODO [#B] TS/JS coverage status sync
 Update the =dev-fkeys.el= header comment (L33) — TS/JS is no longer punted; the cmd-builder at L384 emits vitest/jest. Document the prefer-vitest fallback.
 
-** TODO [#B] heavy-box comment inserts non-comment lines :bug:solo:next:
+** DONE [#B] heavy-box comment inserts non-comment lines :bug:solo:next:
+CLOSED: [2026-06-15 Mon]
+cj/--comment-heavy-box now prefixes the interior empty/text lines with the comment char + suffix (like cj/--comment-box) so they stay valid comments in line-comment languages, and gained the min-length guard (small/negative widths now error cleanly instead of hitting make-string). The two characterization assertions that pinned the broken bare-* lines were updated to the corrected output.
 =modules/custom-comments.el:427= — =cj/--comment-heavy-box= interior/empty lines carry no comment prefix, so in line-comment languages (elisp, Python) C-; C h injects syntax-breaking bare =*...= lines. The existing test characterizes the broken output (asserts =^\*.*\*$=). Prefix interiors like =cj/--comment-box= does; add the missing min-length validation (negative width hits make-string with a raw error); fix the test to assert corrected output. From the 2026-06 config audit.
 
 ** TODO [#B] jumper: register collisions and dead-marker errors :bug:solo: