diff --git a/frontends/monkey/browser.c b/frontends/monkey/browser.c
index 958375486..6b406f552 100644
--- a/frontends/monkey/browser.c
+++ b/frontends/monkey/browser.c
@@ -27,10 +27,13 @@
 #include "utils/log.h"
 #include "utils/messages.h"
 #include "utils/nsurl.h"
+#include "utils/nsoption.h"
 #include "netsurf/mouse.h"
 #include "netsurf/window.h"
 #include "netsurf/browser_window.h"
+#include "netsurf/content_type.h"
 #include "netsurf/plotters.h"
+#include "css/utils.h"
 
 #include "monkey/output.h"
 #include "monkey/browser.h"
@@ -599,6 +602,69 @@ monkey_window_handle_redraw(int argc, char **argv)
 	moutf(MOUT_WINDOW, "REDRAW WIN %d STOP", atoi(argv[2]));
 }
 
+/**
+ * WINDOW DUMP <win> [path]
+ *
+ * Port-local diagnostic. The layout dump goes to a file because it is
+ * hundreds of lines, then the head of it is echoed so the box geometry is
+ * visible without any way to get a file off the ramdisk. The LENGTHS line
+ * carries the inputs every CSS length conversion depends on -- if the DPI or
+ * font sizes come out wrong, every box is wrong and nothing plots.
+ */
+static void
+monkey_window_handle_dump(int argc, char **argv)
+{
+	const char *path = (argc > 3) ? argv[3] : "0:/users/admin/nsbox.txt";
+	struct gui_window *gw;
+	char line[512];
+	int lineno;
+	FILE *f;
+
+	if (argc != 3 && argc != 4) {
+		moutf(MOUT_ERROR, "WINDOW DUMP ARGS BAD");
+		return;
+	}
+
+	gw = monkey_find_window_by_num(atoi(argv[2]));
+	if (gw == NULL) {
+		moutf(MOUT_ERROR, "WINDOW NUM BAD");
+		return;
+	}
+
+	/* Raw css_fixed (22.10), not FIXTOINT: a value that is merely small
+	 * rather than zero has to stay visible. */
+	moutf(MOUT_WINDOW,
+	      "DUMP WIN %s LENGTHS DPI %d FONT_SIZE %d FONT_MIN %d SCALE %d",
+	      argv[2], (int) nscss_screen_dpi, nsoption_int(font_size),
+	      nsoption_int(font_min_size), nsoption_int(scale));
+
+	f = fopen(path, "w");
+	if (f == NULL) {
+		moutf(MOUT_ERROR, "WINDOW DUMP OPEN FAILED %s", path);
+		return;
+	}
+
+	browser_window_debug_dump(gw->bw, f, CONTENT_DEBUG_RENDER);
+	fclose(f);
+
+	f = fopen(path, "r");
+	if (f == NULL) {
+		moutf(MOUT_ERROR, "WINDOW DUMP REOPEN FAILED %s", path);
+		return;
+	}
+
+	for (lineno = 0; lineno < 40; lineno++) {
+		if (fgets(line, sizeof(line), f) == NULL) {
+			break;
+		}
+		line[strcspn(line, "\n")] = '\0';
+		moutf(MOUT_WINDOW, "DUMP WIN %s BOX %s", argv[2], line);
+	}
+	fclose(f);
+
+	moutf(MOUT_WINDOW, "DUMP WIN %s END LINES %d", argv[2], lineno);
+}
+
 static void
 monkey_window_handle_reload(int argc, char **argv)
 {
@@ -721,6 +787,8 @@ monkey_window_handle_command(int argc, char **argv)
 		monkey_window_handle_stop(argc, argv);
 	} else if (strcmp(argv[1], "REDRAW") == 0) {
 		monkey_window_handle_redraw(argc, argv);
+	} else if (strcmp(argv[1], "DUMP") == 0) {
+		monkey_window_handle_dump(argc, argv);
 	} else if (strcmp(argv[1], "RELOAD") == 0) {
 		monkey_window_handle_reload(argc, argv);
 	} else if (strcmp(argv[1], "EXEC") == 0) {