Bug 1843622 - Better fix / workaround for invalid scale in ScaleOffset r=gfx-reviewers,lsalzman

Differential Revision: https://phabricator.services.mozilla.com/D183698

gfx/tests/crashtests/1843622.html

@@ -0,0 +1,11 @@
+<style>
+#a {
+  transform: scale(0) rotate(0grad);
+  will-change: opacity, contents, left, -webkit-transform;
+}
+:not(mpath) {
+  -webkit-text-stroke-width: 1px;
+}
+</style>
+<textarea id="a" autofocus="autofocus">A</textarea>
+

gfx/tests/crashtests/crashtests.list

@@ -227,4 +227,5 @@ load 1802382-1.html
 load 1808830.html
 load 1825450.html
 load 1683679.html
+load 1843622.html
 

gfx/wr/webrender/src/util.rs

@@ -197,13 +197,12 @@ impl ScaleOffset {
     }
 
     pub fn inverse(&self) -> Self {
-        // If either of the scale factors is 0, all we can do is
-        // return identity.
+        // If either of the scale factors is 0, inverse also has scale 0
         // TODO(gw): Consider making this return Option<Self> in future
         //           so that callers can detect and handle when inverse
         //           fails here.
         if self.scale.x.approx_eq(&0.0) || self.scale.y.approx_eq(&0.0) {
-            return ScaleOffset::identity();
+            return ScaleOffset::new(0.0, 0.0, 0.0, 0.0);
         }
 
         ScaleOffset {
@@ -867,11 +866,11 @@ pub mod test {
     fn scale_offset_invalid_scale() {
         let s0 = ScaleOffset::new(0.0, 1.0, 10.0, 20.0);
         let i0 = s0.inverse();
-        assert_eq!(i0, ScaleOffset::identity());
+        assert_eq!(i0, ScaleOffset::new(0.0, 0.0, 0.0, 0.0));
 
         let s1 = ScaleOffset::new(1.0, 0.0, 10.0, 20.0);
         let i1 = s1.inverse();
-        assert_eq!(i1, ScaleOffset::identity());
+        assert_eq!(i1, ScaleOffset::new(0.0, 0.0, 0.0, 0.0));
     }
 
     #[test]