Handle zero-width sized borders without crashing.

webrender/src/batch.rs

@@ -1471,9 +1471,10 @@ impl BrushPrimitive {
                         )
                     }
                     BorderSource::Border { ref handle, .. } => {
-                        let rt_handle = handle
-                            .as_ref()
-                            .expect("bug: render task handle not allocated");
+                        let rt_handle = match *handle {
+                            Some(ref handle) => handle,
+                            None => return None,
+                        };
                         let rt_cache_entry = resource_cache
                             .get_cached_render_task(rt_handle);
                         resource_cache.get_texture_cache_item(&rt_cache_entry.handle)

webrender/src/border.rs

@@ -604,7 +604,7 @@ impl BorderRenderTaskInfo {
         widths: &BorderWidths,
         scale: LayoutToDeviceScale,
         brush_segments: &mut Vec<BrushSegment>,
-    ) -> Self {
+    ) -> Option<Self> {
         let mut border_segments = Vec::new();
 
         let dp_width_top = (widths.top * scale.0).ceil();
@@ -684,6 +684,10 @@ impl BorderRenderTaskInfo {
             dp_size_tl.height.max(dp_size_tr.height) + inner_height + dp_size_bl.height.max(dp_size_br.height),
         );
 
+        if size.width == 0.0 || size.height == 0.0 {
+            return None;
+        }
+
         add_edge_segment(
             LayoutRect::from_floats(
                 rect.origin.x,
@@ -860,10 +864,10 @@ impl BorderRenderTaskInfo {
             brush_segments,
         );
 
-        BorderRenderTaskInfo {
+        Some(BorderRenderTaskInfo {
             border_segments,
             size: size.to_i32(),
-        }
+        })
     }
 
     pub fn build_instances(&self, border: &NormalBorder) -> Vec<BorderInstance> {

webrender/src/prim_store.rs

@@ -1424,39 +1424,39 @@ impl PrimitiveStore {
                 if needs_update {
                     cache_key.scale = scale_au;
 
-                    *task_info = Some(BorderRenderTaskInfo::new(
+                    *task_info = BorderRenderTaskInfo::new(
                         &metadata.local_rect,
                         border,
                         widths,
                         scale,
                         &mut new_segments,
-                    ));
+                    );
                 }
 
-                let task_info = task_info.as_ref().unwrap();
-
-                *handle = Some(frame_state.resource_cache.request_render_task(
-                    RenderTaskCacheKey {
-                        size: DeviceIntSize::zero(),
-                        kind: RenderTaskCacheKeyKind::Border(cache_key.clone()),
-                    },
-                    frame_state.gpu_cache,
-                    frame_state.render_tasks,
-                    None,
-                    false,          // todo
-                    |render_tasks| {
-                        let task = RenderTask::new_border(
-                            task_info.size,
-                            task_info.build_instances(border),
-                        );
-
-                        let task_id = render_tasks.add(task);
-
-                        pic_state.tasks.push(task_id);
-
-                        task_id
-                    }
-                ));
+                *handle = task_info.as_ref().map(|task_info| {
+                    frame_state.resource_cache.request_render_task(
+	                    RenderTaskCacheKey {
+	                        size: DeviceIntSize::zero(),
+	                        kind: RenderTaskCacheKeyKind::Border(cache_key.clone()),
+	                    },
+	                    frame_state.gpu_cache,
+	                    frame_state.render_tasks,
+	                    None,
+	                    false,          // todo
+	                    |render_tasks| {
+	                        let task = RenderTask::new_border(
+	                            task_info.size,
+	                            task_info.build_instances(border),
+	                        );
+
+	                        let task_id = render_tasks.add(task);
+
+	                        pic_state.tasks.push(task_id);
+
+	                        task_id
+	                    }
+	                )
+	            });
 
                 if needs_update {
                     brush.segment_desc = Some(BrushSegmentDescriptor {

wrench/reftests/border/blank.yaml

@@ -0,0 +1,3 @@
+---
+root:
+

wrench/reftests/border/reftest.list

@@ -18,3 +18,4 @@ platform(linux,mac) == border-image.yaml border-image-ref.png
 == border-no-bogus-line.yaml border-no-bogus-line-ref.png
 platform(linux,mac) == dotted-corner-small-radius.yaml dotted-corner-small-radius.png
 == overlapping.yaml overlapping.png
+== zero-width.yaml blank.yaml

wrench/reftests/border/zero-width.yaml

@@ -0,0 +1,12 @@
+root:
+  items:
+    - type: stacking-context
+      bounds: [0, 0, 500, 500]
+      items:
+        - type: border
+          bounds: [ 0, 0, 50, 50 ]
+          width: [ 0, 0, 0, 0 ]
+          border-type: normal
+          style: [ solid, solid, solid, solid ]
+          color: [ blue, blue, blue, blue ]
+