From ad74c5ed7f62913d38543ff6843f38d14689b56d Mon Sep 17 00:00:00 2001
From: Nicholas Marriott <nicholas.marriott@gmail.com>
Date: Mon, 29 Jun 2026 10:20:38 +0100
Subject: [PATCH] Add additional grid consistency checks for macOS.

---
 grid.c | 49 ++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 46 insertions(+), 3 deletions(-)

diff --git a/grid.c b/grid.c
index 44efa574d..28680cbdd 100644
--- a/grid.c
+++ b/grid.c
@@ -18,6 +18,9 @@
 
 #include <sys/types.h>
 
+#ifdef __APPLE__
+#include <assert.h>
+#endif
 #include <stdlib.h>
 #include <string.h>
 
@@ -56,6 +59,28 @@ static const struct grid_cell_entry grid_cleared_entry = {
 	{ .data = { 0, 8, 8, ' ' } }, GRID_FLAG_CLEARED
 };
 
+#ifdef __APPLE__
+static void
+grid_check_lines(struct grid *gd)
+{
+	u_int	i, j;
+
+	for (i = 0; i < gd->hsize + gd->sy; i++) {
+		for (j = i + 1; j < gd->hsize + gd->sy; j++) {
+			if (gd->linedata[i].celldata != NULL)
+				assert(gd->linedata[i].celldata != gd->linedata[j].celldata);
+			if (gd->linedata[i].extddata != NULL)
+				assert(gd->linedata[i].extddata != gd->linedata[j].extddata);
+		}
+	}
+}
+#else
+static void
+grid_check_lines(__unused struct grid *gd)
+{
+}
+#endif
+
 /* Store cell in entry. */
 static void
 grid_store_cell(struct grid_cell_entry *gce, const struct grid_cell *gc,
@@ -287,9 +312,17 @@ grid_set_tab(struct grid_cell *gc, u_int width)
 static void
 grid_free_line(struct grid *gd, u_int py)
 {
-	free(gd->linedata[py].celldata);
-	free(gd->linedata[py].extddata);
-	memset(&gd->linedata[py], 0, sizeof gd->linedata[py]);
+	struct grid_line	*gl = &gd->linedata[py];
+
+#ifdef __APPLE__
+	assert(gl->cellused <= gl->cellsize);
+	assert(gl->extdsize == 0 || gl->extddata != NULL);
+	assert(gl->cellsize == 0 || gl->celldata != NULL);
+#endif
+
+	free(gl->celldata);
+	free(gl->extddata);
+	memset(gl, 0, sizeof *gl);
 }
 
 /* Free several lines. */
@@ -449,6 +482,8 @@ grid_scroll_history(struct grid *gd, u_int bg)
 	gd->linedata[gd->hsize].time = current_time;
 	gd->hsize++;
 	gd->scroll_added++;
+
+	grid_check_lines(gd);
 }
 
 /* Clear the history. */
@@ -498,6 +533,8 @@ grid_scroll_history_region(struct grid *gd, u_int upper, u_int lower, u_int bg)
 	gd->hscrolled++;
 	gd->hsize++;
 	gd->scroll_added++;
+
+	grid_check_lines(gd);
 }
 
 /* Expand line to fit to cell. */
@@ -759,6 +796,8 @@ grid_move_lines(struct grid *gd, u_int dy, u_int py, u_int ny, u_int bg)
 	}
 	if (py != 0 && (py < dy || py >= dy + ny))
 		gd->linedata[py - 1].flags &= ~GRID_LINE_WRAPPED;
+
+	grid_check_lines(gd);
 }
 
 /* Move a group of cells. */
@@ -1248,6 +1287,8 @@ grid_duplicate_lines(struct grid *dst, u_int dy, struct grid *src, u_int sy,
 		sy++;
 		dy++;
 	}
+
+	grid_check_lines(dst);
 }
 
 /* Mark line as dead. */
@@ -1544,6 +1585,8 @@ grid_reflow(struct grid *gd, u_int sx)
 	gd->linedata = target->linedata;
 	free(target);
 	gd->scroll_generation++;
+
+	grid_check_lines(gd);
 }
 
 /* Convert to position based on wrapped lines. */