Set Resize Observer's last reported size to -1x-1 initially.

It used to be 0x0, but changed per the CSSWG resolution:
w3c/csswg-drafts#3664
Sits behind pref `dom.resize_observer.last_reported_size_invalid`.

Differential Revision: https://phabricator.services.mozilla.com/D155710

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1785305
gecko-commit: 14e81415cae10e0699145eb052b49d369f5ee427
gecko-reviewers: emilio

resize-observer/notify.html

@@ -194,27 +194,38 @@
 
 function test6() {
   let helper = new ResizeTestHelper(
-    "test6: inline element does not notify",
+    "test6: inline element notifies once with 0x0.",
   [
     {
       setup: observer => {
         observer.observe(inline);
-        observer.observe(t1);
-        t1.style.width = "66px";
-        inline.style.width = "66px";
       },
       notify: (entries, observer) => {
-        assert_equals(entries.length, 1, "inline elements must not trigger notifications");
-        assert_equals(entries[0].target, t1, "inline elements must not trigger notifications");
+        assert_equals(entries.length, 1, "observing inline element triggers notification");
+        assert_equals(entries[0].target, inline, "observing inline element triggers notification");
+        assert_equals(entries[0].contentRect.width, 0);
+        assert_equals(entries[0].contentRect.height, 0);
         return true; // Delay next step
       }
     },
+    {
+      setup: observer => {
+        inline.style.width = "66px";
+      },
+      notify: (entries, observer) => {
+        assert_unreached("resizing inline element should not cause resize notifications");
+      },
+      timeout: () => {
+        // expected
+      }
+    },
     { // "inline element that becomes block should notify",
       setup: observer => {
         inline.style.display = "block";
       },
       notify: (entries, observer) => {
-        assert_equals(entries[0].target, inline);
+        assert_equals(entries.length, 1, "inline element becoming a non-zero sized block triggers a notification");
+        assert_equals(entries[0].target, inline, "inline element becoming a non-zero sized block triggers a notification");
       }
     }
   ]);

resize-observer/observe.html

@@ -719,16 +719,26 @@
 
   let helper = new ResizeTestHelper(
     // See: https://drafts.csswg.org/resize-observer/#intro.
-    "test16: observations do not fire for non-replaced inline elements",
+    "test16: observations fire once with 0x0 size for non-replaced inline elements",
   [
     {
       setup: observer => {
         observer.observe(t);
       },
       notify: entries => {
-        assert_unreached("No observation should fire for non box element")
-      },
-      timeout: () => {}
+        assert_equals(entries.length, 1, "1 pending notification");
+        assert_equals(entries[0].target, t, "target is t");
+        assert_equals(entries[0].contentRect.width, 0, "target width");
+        assert_equals(entries[0].contentRect.height, 0, "target height");
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 0,
+                      "target content-box inline size");
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 0,
+                      "target content-box block size");
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 0,
+                      "target border-box inline size");
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 0,
+                      "target border-box block size");
+      }
     }
   ]);
 

resize-observer/svg.html

@@ -345,21 +345,21 @@
         observer.observe(view);
       },
       notify: (entries, observer) => {
-        assert_unreached("no entries should be observed on <view> Element");
-      },
-      timeout: () => {
-        // expected
+        assert_equals(entries.length, 1);
+        assert_equals(entries[0].target, view);
+        assert_equals(entries[0].contentRect.width, 0);
+        assert_equals(entries[0].contentRect.height, 0);
       }
     },
     {
       setup: observer => {
         observer.observe(stop);
       },
       notify: (entries, observer) => {
-        assert_unreached("no entries should be observed on <stop> Element");
-      },
-      timeout: () => {
-        // expected
+        assert_equals(entries.length, 1);
+        assert_equals(entries[0].target, stop);
+        assert_equals(entries[0].contentRect.width, 0);
+        assert_equals(entries[0].contentRect.height, 0);
       }
     },
   ]);