Bug 1891322 - more robust solution for NaN offset in gradients r=gfx-reviewers,lsalzman

Differential Revision: https://phabricator.services.mozilla.com/D207904

Author: ladyhavoc

webrender_api/src/gradient_builder.rs

@@ -132,10 +132,6 @@ impl GradientBuilder {
         let first = *stops.first().unwrap();
         let last = *stops.last().unwrap();
 
-        // Express the assertion so that if one of the offsets is NaN, we don't panic
-        // and instead take the branch that handles degenerate gradients.
-        assert!(!(first.offset > last.offset));
-
         let stops_delta = last.offset - first.offset;
 
         if stops_delta > 0.000001 {
@@ -144,6 +140,14 @@ impl GradientBuilder {
             }
 
             (first.offset, last.offset)
+        } else if stops_delta.is_nan() {
+            // We have no good way to render a NaN offset, but make something
+            // that is at least renderable.
+            stops.clear();
+            stops.push(di::GradientStop { color: last.color, offset: 0.0, });
+            stops.push(di::GradientStop { color: last.color, offset: 1.0, });
+
+            (0.0, 1.0)
         } else {
             // We have a degenerate gradient and can't accurately transform the stops
             // what happens here depends on the repeat behavior, but in any case