diff --git a/webrender/src/frame_builder.rs b/webrender/src/frame_builder.rs
index bdad0d3dd6..5ba50b80e6 100644
--- a/webrender/src/frame_builder.rs
+++ b/webrender/src/frame_builder.rs
@@ -2037,7 +2037,8 @@ impl<'a> LayerRectCalculationAndCullingPass<'a> {
                 let clip_task = RenderTask::new_mask(cache_key,
                                                      mask_rect,
                                                      &self.current_clip_stack,
-                                                     extra);
+                                                     extra,
+                                                     prim_screen_rect);
                 let render_tasks = &mut self.render_tasks;
                 prim_metadata.clip_task_id = clip_task.map(|clip_task| {
                     render_tasks.add(clip_task)
diff --git a/webrender/src/render_task.rs b/webrender/src/render_task.rs
index 18106ed726..cc64c857b3 100644
--- a/webrender/src/render_task.rs
+++ b/webrender/src/render_task.rs
@@ -244,7 +244,8 @@ impl RenderTask {
     pub fn new_mask(key: Option<ClipId>,
                     task_rect: DeviceIntRect,
                     raw_clips: &[ClipWorkItem],
-                    extra_clip: Option<ClipWorkItem>)
+                    extra_clip: Option<ClipWorkItem>,
+                    prim_rect: DeviceIntRect)
                     -> Option<RenderTask> {
         // Filter out all the clip instances that don't contribute to the result
         let mut inner_rect = Some(task_rect);
@@ -283,13 +284,20 @@ impl RenderTask {
         //           In the future, we'll expand this to handle the
         //           more complex types of clip mask geometry.
         let mut geometry_kind = MaskGeometryKind::Default;
-        if inner_rect.is_some() && clips.len() == 1 {
-            let (_, ref info) = clips[0];
-            if info.border_corners.is_empty() &&
-               info.image.is_none() &&
-               info.complex_clip_range.get_count() == 1 &&
-               info.layer_clip_range.get_count() == 0 {
-                geometry_kind = MaskGeometryKind::CornersOnly;
+        if let Some(inner_rect) = inner_rect {
+            // If the inner rect completely contains the primitive
+            // rect, then this mask can't affect the primitive.
+            if inner_rect.contains_rect(&prim_rect) {
+                return None;
+            }
+            if clips.len() == 1 {
+                let (_, ref info) = clips[0];
+                if info.border_corners.is_empty() &&
+                   info.image.is_none() &&
+                   info.complex_clip_range.get_count() == 1 &&
+                   info.layer_clip_range.get_count() == 0 {
+                    geometry_kind = MaskGeometryKind::CornersOnly;
+                }
             }
         }