--- a/sway/desktop/xdg_shell.c
+++ b/sway/desktop/xdg_shell.c
@@ -1,6 +1,8 @@
 #include <float.h>
+#include <inttypes.h>
 #include <stdbool.h>
 #include <stdlib.h>
+#include <time.h>
 #include <wayland-server-core.h>
 #include <wlr/types/wlr_xdg_shell.h>
 #include <wlr/types/wlr_xdg_toplevel_tag_v1.h>
@@ -293,6 +295,32 @@
 		wl_container_of(listener, xdg_shell_view, commit);
 	struct sway_view *view = &xdg_shell_view->view;
 	struct wlr_xdg_surface *xdg_surface = view->wlr_xdg_toplevel->base;
+	// CONTRACT: opt-out permits a same-workload trace-overhead comparison;
+	// the switch changes only this log site, not the backend scheduler.
+	if (getenv("CPI_SURFACE_TRACE") == NULL ||
+			getenv("CPI_SURFACE_TRACE")[0] != '0') {
+	struct wlr_surface *surface = xdg_surface->surface;
+	struct timespec ordering_now;
+	clock_gettime(CLOCK_MONOTONIC, &ordering_now);
+	// WHY: this callback observes the applied Wayland input state, independently
+	// of output submission and presentation. Surface IDs expire at destruction.
+	// CONTRACT: record state only; never alter scheduling or current.input.
+	// INVARIANT: an input-only commit may change hit without a new client buffer.
+	const pixman_box32_t *input_bounds = pixman_region32_extents(&surface->input_region);
+	sway_log(SWAY_INFO, "ORDER_SURFACE_COMMIT ns=%" PRIu64
+		" surface=%p resource=%u seq=%" PRIu32 " committed=%" PRIu32
+		" input_changed=%d buffer_changed=%d input_rects=%d"
+		" input_bounds=%d,%d,%d,%d width=%d height=%d sample_accepts=%d",
+		(uint64_t)ordering_now.tv_sec * UINT64_C(1000000000) + ordering_now.tv_nsec,
+		(void *)surface, wl_resource_get_id(surface->resource), surface->current.seq,
+		surface->current.committed,
+		!!(surface->current.committed & WLR_SURFACE_STATE_INPUT_REGION),
+		!!(surface->current.committed & WLR_SURFACE_STATE_BUFFER),
+		pixman_region32_n_rects(&surface->input_region),
+		input_bounds->x1, input_bounds->y1, input_bounds->x2, input_bounds->y2,
+		surface->current.width, surface->current.height,
+		wlr_surface_point_accepts_input(surface, 100, 150));
+	}
 
 	if (xdg_surface->initial_commit) {
 		if (view->xdg_decoration != NULL) {