fix: include child subtree in entry-empty-p search bound (upstream #13)

kqr (2019-07-22) reported that drill entries whose answer lives inside a child sub-heading were silently skipped. Their example: a question in the heading text and the answer under `** The Answer`. The function returned t (empty) for such entries, so they never got presented during drill sessions.

The cause is `(outline-next-heading)` in `org-drill-entry-empty-p`. That primitive lands on the first heading at any level, including children. So the search range was metadata-end up to the child's heading line, which excluded the child's body. Bodies that lived in child sub-headings never got searched.

I switched the bound to `(org-end-of-subtree t t)`, which covers the whole subtree of the current heading and degrades gracefully at the last heading in the buffer. The reporter suggested `outline-forward-same-level`, but that primitive errors at the last sibling, which would be its own regression. `org-end-of-subtree` is the canonical Emacs idiom for this kind of bound and handles end-of-buffer correctly.

I added `tests/test-org-drill-entry-empty-p.el` with 6 ERT tests across Normal, Boundary (kqr's exact fixture), and edge categories. The two regression tests fail at HEAD before the fix and pass after.

One semantic note worth flagging: any subtree content now counts as non-empty, including bare child headings with no body of their own. The bug report is silent on that case and I expect it to be rare in practice. If anyone reports the new behavior as a regression, the fix would be to filter heading lines out of the graphical-character search.

1 files changed, 8 insertions, 2 deletions

diff --git a/org-drill.el b/org-drill.el
index 07de25d..c3a73eb 100644
--- a/org-drill.el
+++ b/org-drill.el
@@ -2157,11 +2157,17 @@ Note: does not actually alter the item."
 
 ;; This version is about 5x faster than the old version, above.
 (defun org-drill-entry-empty-p ()
-  "Return non-nil if the current entry is empty."
+  "Return non-nil if the current entry is empty.
+
+The search bound covers the whole subtree, so an entry whose answer
+lives inside a child sub-heading is correctly reported as non-empty
+\(upstream issue #13).  `org-end-of-subtree' is used in place of
+`outline-next-heading' because the latter lands on the first child
+heading, which truncates the search range before the child's body."
   (save-excursion
     (org-back-to-heading t)
     (let ((lim (save-excursion
-                 (outline-next-heading) (point))))
+                 (org-end-of-subtree t t) (point))))
       (if (fboundp 'org-end-of-meta-data-and-drawers)
           (org-end-of-meta-data-and-drawers) ; function removed Feb 2015
         (org-end-of-meta-data t))