--- a/gfx/wr/webrender/src/frame_builder.rs
+++ b/gfx/wr/webrender/src/frame_builder.rs
@@ -433,16 +433,17 @@ impl FrameBuilder {
                 data_stores,
                 scratch,
                 tile_cache_logger,
             );
         }
 
         let pic = &mut scene.prim_store.pictures[scene.root_pic_index.0];
         pic.restore_context(
+            ROOT_SURFACE_INDEX,
             prim_list,
             pic_context,
             pic_state,
             &mut frame_state,
         );
 
         frame_state.pop_dirty_region();
 

--- a/gfx/wr/webrender/src/picture.rs
+++ b/gfx/wr/webrender/src/picture.rs
@@ -3272,16 +3272,20 @@ pub struct PicturePrimitive {
     pub segments_are_valid: bool,
 
     /// If Some(..) the tile cache that is associated with this picture.
     #[cfg_attr(feature = "capture", serde(skip))] //TODO
     pub tile_cache: Option<Box<TileCacheInstance>>,
 
     /// The config options for this picture.
     pub options: PictureOptions,
+
+    /// Keep track of the number of render tasks dependencies to pre-allocate
+    /// the dependency array next frame.
+    num_render_tasks: usize,
 }
 
 impl PicturePrimitive {
     pub fn print<T: PrintTreePrinter>(
         &self,
         pictures: &[Self],
         self_index: PictureIndex,
         pt: &mut T,
@@ -3414,16 +3418,17 @@ impl PicturePrimitive {
             is_backface_visible: flags.contains(PrimitiveFlags::IS_BACKFACE_VISIBLE),
             requested_raster_space,
             spatial_node_index,
             estimated_local_rect: LayoutRect::zero(),
             precise_local_rect: LayoutRect::zero(),
             tile_cache,
             options,
             segments_are_valid: false,
+            num_render_tasks: 0,
         }
     }
 
     /// Gets the raster space to use when rendering the picture.
     /// Usually this would be the requested raster space. However, if the
     /// picture's spatial node or one of its ancestors is being pinch zoomed
     /// then we round it. This prevents us rasterizing glyphs for every minor
     /// change in zoom level, as that would be too expensive.
@@ -3456,16 +3461,19 @@ impl PicturePrimitive {
         frame_context: &FrameBuildingContext,
         scratch: &mut PrimitiveScratchBuffer,
         tile_cache_logger: &mut TileCacheLogger,
     ) -> Option<(PictureContext, PictureState, PrimitiveList)> {
         if !self.is_visible() {
             return None;
         }
 
+        let task_id = frame_state.surfaces[parent_surface_index.0].render_tasks.unwrap().port;
+        frame_state.render_tasks[task_id].children.reserve(self.num_render_tasks);
+
         // Extract the raster and surface spatial nodes from the raster
         // config, if this picture establishes a surface. Otherwise just
         // pass in the spatial node indices from the parent context.
         let (raster_spatial_node_index, surface_spatial_node_index, surface_index, inflation_factor) = match self.raster_config {
             Some(ref raster_config) => {
                 let surface = &frame_state.surfaces[raster_config.surface_index.0];
 
                 (
@@ -4220,26 +4228,30 @@ impl PicturePrimitive {
 
         let prim_list = mem::replace(&mut self.prim_list, PrimitiveList::empty());
 
         Some((context, state, prim_list))
     }
 
     pub fn restore_context(
         &mut self,
+        parent_surface_index: SurfaceIndex,
         prim_list: PrimitiveList,
         context: PictureContext,
         state: PictureState,
         frame_state: &mut FrameBuildingState,
     ) {
         // Pop any dirty regions this picture set
         for _ in 0 .. context.dirty_region_count {
             frame_state.pop_dirty_region();
         }
 
+        let task_id = frame_state.surfaces[parent_surface_index.0].render_tasks.unwrap().port;
+        self.num_render_tasks = frame_state.render_tasks[task_id].children.len();
+
         self.prim_list = prim_list;
         self.state = Some(state);
     }
 
     pub fn take_state(&mut self) -> PictureState {
         self.state.take().expect("bug: no state present!")
     }
 

--- a/gfx/wr/webrender/src/prim_store/mod.rs
+++ b/gfx/wr/webrender/src/prim_store/mod.rs
@@ -2727,16 +2727,17 @@ impl PrimitiveStore {
                     data_stores,
                     scratch,
                     tile_cache_log,
                 );
 
                 // Restore the dependencies (borrow check dance)
                 self.pictures[pic_context_for_children.pic_index.0]
                     .restore_context(
+                        pic_context.surface_index,
                         prim_list,
                         pic_context_for_children,
                         pic_state_for_children,
                         frame_state,
                     );
 
                 is_passthrough
             }