Round getBoundingClientRect()'s height and Element.scrollTop to avoid mis-comparison between double values and integer values.

The target element (top_box) height is defined by '60vh', the scroll
position also depends on vh-units values, thus these values would be
fractional on certain browser window sizes.

(It can be observable with the default headless mode browser window size,
1366x768.)

Differential Revision: https://phabricator.services.mozilla.com/D249193

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1966276
gecko-commit: 4fa6a18a08006b4fc3e852f5ee1faf912d709957
gecko-reviewers: emilio

Author: hiikezoe

css/css-scroll-snap/scroll-initial-target/scroll-initial-target-with-text-fragment-navigation-target.html

@@ -57,11 +57,12 @@
       const expected_scroll_top = document.scrollingElement.scrollHeight -
         document.scrollingElement.clientHeight;
 
-      const scroll_start_target_top = top_box.getBoundingClientRect().height;
+      const scroll_start_target_top = Math.round(top_box.getBoundingClientRect().height);
 
-      if (document.scrollingElement.scrollTop == scroll_start_target_top) {
+      const actual_scroll_top = Math.round(document.scrollingElement.scrollTop);
+      if (actual_scroll_top == scroll_start_target_top) {
         scroll_position = "AT_SCROLL_START_TARGET";
-      } else if (document.scrollingElement.scrollTop == expected_scroll_top) {
+      } else if (actual_scroll_top == expected_scroll_top) {
         scroll_position = "AT_TEXT_FRAGMENT";
       }