The problem appears to occur in diff_step() and longestCommonSequence(),
which produces a triple of 24 deletions and 26 insertions from line 3313, rather
than the expected two hunks of 1 deletion and 2 insertions diff(1) produces. But
it could result from an earlier problem in break_into_lines() or even
compare_dline() because passing the -w flag produces the correct output (but
there are no whitespace changes between the misreported lines). Interestingly,
Fossil's side-by-side diff -y (or --tk) produces the correct diff too.

• from file: https://fnc.bsdbox.org/file?name=src/fnc.c&ci=5dddbafd09d144c3
• to file: https://itac.bsdbox.org/fnc.c

Below are the relevant hunks from the diff.

fossil diff:

@@ -3309,34 +3306,36 @@
 			return RC(FSL_RC_ERROR, "%s", "view_open");
 		rc = open_timeline_view(new, 0, "/", glob);
 		if (rc) {
 			if (rc != FSL_RC_BREAK)
 				return rc;
-			fnc_print_msg(view, "-- no matching commits --", true);
-			fcli_err_reset();
-			rc = 0;
-			break;
-		}
-		view->active = false;
-		new->active = true;
-		if (view_is_parent(view)) {
-			rc = view_close_child(view);
-			if (rc)
-				return rc;
-			view_set_child(view, new);
-			view->focus_child = true;
-		} else
-			*new_view = new;
-		break;
-	}
-	case 't':
-		if (s->selected_commit == NULL)
-			break;
-		if (!fsl_rid_is_a_checkin(fcli_cx(),
-		    s->selected_commit->commit->rid)) {
-			fnc_print_msg(view,
-			    "-- tree requires check-in artifact --", true);
+			fnc_print_msg(view, "-- no matching commits --", true,
+			    true, true);
+			fcli_err_reset();
+			rc = 0;
+			break;
+		}
+		view->active = false;
+		new->active = true;
+		if (view_is_parent(view)) {
+			rc = view_close_child(view);
+			if (rc)
+				return rc;
+			view_set_child(view, new);
+			view->focus_child = true;
+		} else
+			*new_view = new;
+		break;
+	}
+	case 't':
+		if (s->selected_commit == NULL)
+			break;
+		if (!fsl_rid_is_a_checkin(fcli_cx(),
+		    s->selected_commit->commit->rid)) {
+			fnc_print_msg(view,
+			    "-- tree requires check-in artifact --", true,
+			    true, true);
 			fcli_err_reset();
 			rc = 0;
 			break;
 		}
 		rc = request_new_view(&tree_view, view, FNC_VIEW_TREE);
@@ -3749,12 +3748,11 @@

diff(1): diff -U5^{^}

@@ -3309,11 +3306,12 @@
 			return RC(FSL_RC_ERROR, "%s", "view_open");
 		rc = open_timeline_view(new, 0, "/", glob);
 		if (rc) {
 			if (rc != FSL_RC_BREAK)
 				return rc;
-			fnc_print_msg(view, "-- no matching commits --", true);
+			fnc_print_msg(view, "-- no matching commits --", true,
+			    true, true);
 			fcli_err_reset();
 			rc = 0;
 			break;
 		}
 		view->active = false;
@@ -3332,11 +3330,12 @@
 		if (s->selected_commit == NULL)
 			break;
 		if (!fsl_rid_is_a_checkin(fcli_cx(),
 		    s->selected_commit->commit->rid)) {
 			fnc_print_msg(view,
-			    "-- tree requires check-in artifact --", true);
+			    "-- tree requires check-in artifact --", true,
+			    true, true);
 			fcli_err_reset();
 			rc = 0;
 			break;
 		}
 		rc = request_new_view(&tree_view, view, FNC_VIEW_TREE);
@@ -3749,12 +3748,11 @@

^{^}for consistency with Fossil's default 5 context line unified diff