If a pane is outside the window, treat as obscured.

screen-write.c

@@ -191,7 +191,15 @@ screen_write_pane_is_obscured(struct screen_write_ctx *ctx)
 	}
 	ctx->flags |= SCREEN_WRITE_CHECKED_IF_OBSCURED;
 
-        while ((wp = TAILQ_PREV(wp, window_panes, zentry)) != NULL) {
+	if (ctx->wp->xoff < 0 ||
+	    ctx->wp->yoff < 0 ||
+	    ctx->wp->xoff + ctx->wp->sx >= ctx->wp->window->sx ||
+	    ctx->wp->yoff + ctx->wp->sy >= ctx->wp->window->sy) {
+		ctx->flags |= SCREEN_WRITE_OBSCURED;
+		return (1);
+	}
+
+	while ((wp = TAILQ_PREV(wp, window_panes, zentry)) != NULL) {
                 if ((wp->flags & PANE_FLOATING) &&
                     ((wp->yoff >= ctx->wp->yoff &&
                     wp->yoff <= ctx->wp->yoff + (int)ctx->wp->sy) ||