fix: skip LaTeX preview on TTY frames (upstream #44)

Issue #44 (2021): running org-drill in a TTY emacsclient (the
reporter mentioned tmux) raised "Window system frame should be
used" because LaTeX preview helpers (org-latex-preview,
org--latex-preview-region) require a window system and weren't
guarded.

Wrapped both call sites with (when (display-graphic-p) ...).

- org-drill--show-latex-fragments: now a silent no-op on TTY
- present-default-answer's clear-and-preview block: same guard

LaTeX previews are inherently graphical.  The right behavior on
TTY is to skip the preview rather than crash the session — TTY
users still see the underlying source text just fine.

1 files changed, 49 insertions, 0 deletions

diff --git a/tests/test-org-drill-tty-safe.el b/tests/test-org-drill-tty-safe.el
new file mode 100644
index 0000000..7ec5cf2
--- /dev/null
+++ b/tests/test-org-drill-tty-safe.el
@@ -0,0 +1,49 @@
+;;; test-org-drill-tty-safe.el --- Regression for TTY/no-window-system error  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Upstream issue #44 (2021-10).  Running org-drill in a TTY emacsclient
+;; (e.g., inside tmux) produced "error: Window system frame should be
+;; used" because the LaTeX preview helpers (`org-latex-preview',
+;; `org--latex-preview-region') require a window-system frame and
+;; weren't guarded.
+;;
+;; Fix: gate `org-drill--show-latex-fragments' on `display-graphic-p',
+;; making it a no-op on TTY.  LaTeX previews are inherently graphical
+;; — the right behavior in TTY is to skip the preview rather than
+;; crash the session.
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+(require 'org)
+(require 'org-drill)
+
+;;;; Regression — #44
+
+(ert-deftest test-show-latex-fragments-noop-on-tty ()
+  "On a non-graphic display, the helper should be a silent no-op
+rather than calling org-latex-preview (which requires a window
+system frame)."
+  (let ((latex-preview-called nil))
+    (cl-letf (((symbol-function 'display-graphic-p) (lambda () nil))
+              ((symbol-function 'org-clear-latex-preview)
+               (lambda () (setq latex-preview-called 'cleared)))
+              ((symbol-function 'org-latex-preview)
+               (lambda (&rest _) (setq latex-preview-called 'displayed))))
+      (org-drill--show-latex-fragments)
+      (should (null latex-preview-called)))))
+
+(ert-deftest test-show-latex-fragments-runs-on-graphic-display ()
+  "On a graphic display, the helper still calls through to org-latex-preview."
+  (let ((latex-preview-called nil))
+    (cl-letf (((symbol-function 'display-graphic-p) (lambda () t))
+              ((symbol-function 'org-clear-latex-preview) #'ignore)
+              ((symbol-function 'org-latex-preview)
+               (lambda (&rest _) (setq latex-preview-called t))))
+      (org-drill--show-latex-fragments)
+      (should latex-preview-called))))
+
+(provide 'test-org-drill-tty-safe)
+
+;;; test-org-drill-tty-safe.el ends here