Add a new test for tall screens; draw outside of scanline range.

This exposes a bug in the current code base.  That is, we
do not scan beyond the 32 scan lines currently given.

That means if we have a screen taller than 1024 pixels, we will
not scan every block fully, and so we can miss changes.

This test exposes that issue, and currently fails.

src/tests/tests.c

@@ -203,6 +203,7 @@ void test_tallscreen(xdummy_t *xdummy, gconstpointer user_data)
     snprintf(buf, sizeof(buf), "xrandr --display :%s -s 3840x2160", xdummy->display);
     system(buf);
 
+    /* Test basic drawing */
     snprintf(buf, sizeof(buf), ":%s", xdummy->display);
     if (xcbtest_draw_grid(buf)) {
         g_warning("Could not draw the grid");
@@ -213,6 +214,17 @@ void test_tallscreen(xdummy_t *xdummy, gconstpointer user_data)
         check_screenshot(&test, &server, xdummy, buf);
     }
 
+    /* Test drawing to the very bottom of the screen */
+    snprintf(buf, sizeof(buf), ":%s", xdummy->display);
+    if (xcbtest_draw_at_bottom(buf)) {
+        g_warning("Could not draw at the bottom");
+        g_test_fail();
+    }
+    else {
+        snprintf(buf, sizeof(buf), "%s/expected.grid.bottom.3840x2160.ppm", PATH_TO_TEST_SRC);
+        check_screenshot(&test, &server, xdummy, buf);
+    }
+
     test_common_stop(&test, &server);
 }
 

src/tests/xcbtest.c

@@ -124,3 +124,45 @@ int xcbtest_draw_grid(const char *display)
 
     return 0;
 }
+
+int xcbtest_draw_at_bottom(const char *display)
+{
+    uint32_t red_pixel;
+
+    xcb_connection_t *c;
+    xcb_screen_t *screen;
+    xcb_gcontext_t red_fg;
+
+    xcb_void_cookie_t cookie;
+
+    xcb_rectangle_t red_rectangle;
+
+    /* Open the connection to the X server */
+    c = xcb_connect(display, NULL);
+    if (xcb_connection_has_error(c))
+        return 1;
+
+    /* Get the first screen */
+    screen = xcb_setup_roots_iterator(xcb_get_setup(c)).data;
+
+    red_fg = xcb_generate_id(c);
+    lookup_color(c, screen, "red", &red_pixel);
+    cookie = xcb_create_gc_checked(c, red_fg, screen->root, XCB_GC_FOREGROUND, &red_pixel);
+    xcb_request_check(c, cookie);
+
+    red_rectangle.width = screen->width_in_pixels;
+    red_rectangle.height = 8;
+    red_rectangle.x = 0;
+    red_rectangle.y = screen->height_in_pixels - red_rectangle.height * 2;
+
+    /* We draw the rectangles */
+    cookie = xcb_poly_fill_rectangle_checked(c, screen->root, red_fg, 1, &red_rectangle);
+    xcb_request_check(c, cookie);
+
+    /* We flush the request */
+    xcb_flush(c);
+
+    xcb_disconnect(c);
+
+    return 0;
+}

src/tests/xcbtest.h

@@ -26,5 +26,6 @@
 **  Prototypes
 **--------------------------------------------------------------------------*/
 int xcbtest_draw_grid(const char *display);
+int xcbtest_draw_at_bottom(const char *display);
 
 #endif