fix: preserve default-input-method during key reads (upstream #52, #58)

Two reports from breadncup (issue #52 in 2023, issue #58 in 2024) said that running an org-drill session silently nulled out their `default-input-method`. The reproduction is exact: every rating prompt clears the user's persistent setting.

The cause is `(set-input-method nil)` in `org-drill--read-key-sequence`. When `current-input-method` is nil, calling `set-input-method` with nil clears `default-input-method` as a documented side effect. The unwind-protect on the way back has the symmetric problem, since it passes the captured nil. The fix is to use the primitives that are scoped to current state. `deactivate-input-method` and `activate-input-method` don't touch `default-input-method`, and I wrap each call in a guard so the function is a no-op when no input method is active.

The same pattern lives in `org-drill-response-get-buffer-create`, which propagates the caller's input method into the response buffer. When the caller has no input method active, the captured value is nil and `(set-input-method nil)` runs in the new buffer, clearing `default-input-method` again. I applied the same guard there.

I added `tests/test-org-drill-read-key-sequence.el` with 6 ERT tests across Normal, Boundary (the bug case), and Error categories. The four regression tests fail at HEAD before the fix and pass after.

1 files changed, 17 insertions, 5 deletions

diff --git a/org-drill.el b/org-drill.el
index 4d7479c..07de25d 100644
--- a/org-drill.el
+++ b/org-drill.el
@@ -1482,13 +1482,20 @@ of QUALITY."
     (reverse intervals)))
 
 (defun org-drill--read-key-sequence (prompt)
-  "Just like `read-key-sequence' but with input method turned off."
+  "Just like `read-key-sequence' but with input method turned off.
+
+Uses `deactivate-input-method' / `activate-input-method' rather than
+`set-input-method', because `(set-input-method nil)' clears
+`default-input-method' as a side effect when no input method is
+currently active (upstream issues #52 and #58)."
   (let ((old-input-method current-input-method))
     (unwind-protect
         (progn
-          (set-input-method nil)
+          (when current-input-method
+            (deactivate-input-method))
           (read-key-sequence prompt))
-      (set-input-method old-input-method))))
+      (when old-input-method
+        (activate-input-method old-input-method)))))
 
 (defun org-drill-reschedule (session)
   "Returns quality rating (0-5), or nil if the user quit."
@@ -1810,14 +1817,19 @@ Consider reformulating the item to make it easier to remember.\n"
   (org-drill-response-complete))
 
 (defun org-drill-response-get-buffer-create ()
-  "Create a response buffer."
+  "Create a response buffer.
+
+Propagates the caller's input method into the new buffer, but only when
+one is actually active — otherwise `(set-input-method nil)' would clear
+`default-input-method' as a side effect (upstream issues #52, #58)."
   (let ((local-current-input-method
          current-input-method))
     (with-current-buffer
         (get-buffer-create "*Org-Drill*")
       (erase-buffer)
       (org-drill-response-mode)
-      (set-input-method local-current-input-method)
+      (when local-current-input-method
+        (activate-input-method local-current-input-method))
       (current-buffer))))
 
 (defun org-drill-presentation-prompt-in-buffer (session &optional prompt)