Bug 1563993 - WR chase into clips r=jrmuizel

it's very helpful to see the list of clips and the way they affect a chased primitive
Example:
```
    building clip chain instance with local rect TypedRect(1561.0×1968.0 at (-300.0,-300.0))
        clip Rectangle(3840.0×1874.0, Clip) at (0.0,0.0) in space SpatialNodeIndex(1)
        flags (empty), resulted in Partial
        clip Rectangle(3840.0×1874.0, Clip) at (0.0,0.0) in space SpatialNodeIndex(2)
        flags (empty), resulted in Partial
```

Differential Revision: https://phabricator.services.mozilla.com/D37137

--HG--
extra : moz-landing-system : lando

gfx/wr/webrender/src/clip.rs

@@ -297,6 +297,20 @@ impl ClipSpaceConversion {
             )
         }
     }
+
+    fn to_flags(&self) -> ClipNodeFlags {
+        match *self {
+            ClipSpaceConversion::Local => {
+                ClipNodeFlags::SAME_SPATIAL_NODE | ClipNodeFlags::SAME_COORD_SYSTEM
+            }
+            ClipSpaceConversion::ScaleOffset(..) => {
+                ClipNodeFlags::SAME_COORD_SYSTEM
+            }
+            ClipSpaceConversion::Transform(..) => {
+                ClipNodeFlags::empty()
+            }
+        }
+    }
 }
 
 // Temporary information that is cached and reused
@@ -322,17 +336,7 @@ impl ClipNodeInfo {
     ) -> Option<ClipNodeInstance> {
         // Calculate some flags that are required for the segment
         // building logic.
-        let mut flags = match self.conversion {
-            ClipSpaceConversion::Local => {
-                ClipNodeFlags::SAME_SPATIAL_NODE | ClipNodeFlags::SAME_COORD_SYSTEM
-            }
-            ClipSpaceConversion::ScaleOffset(..) => {
-                ClipNodeFlags::SAME_COORD_SYSTEM
-            }
-            ClipSpaceConversion::Transform(..) => {
-                ClipNodeFlags::empty()
-            }
-        };
+        let mut flags = self.conversion.to_flags();
 
         // Some clip shaders support a fast path mode for simple clips.
         // For now, the fast path is only selected if:
@@ -817,11 +821,15 @@ impl ClipStore {
         world_rect: &WorldRect,
         clip_data_store: &mut ClipDataStore,
         request_resources: bool,
+        is_chased: bool,
     ) -> Option<ClipChainInstance> {
         let local_clip_rect = match self.active_local_clip_rect {
             Some(rect) => rect,
             None => return None,
         };
+        if is_chased {
+            println!("\tbuilding clip chain instance with local rect {:?}", local_prim_rect);
+        }
 
         let local_bounding_rect = local_prim_rect.intersection(&local_clip_rect)?;
         let pic_clip_rect = prim_to_pic_mapper.map(&local_bounding_rect)?;
@@ -860,6 +868,11 @@ impl ClipStore {
                 }
             };
 
+            if is_chased {
+                println!("\t\tclip {:?} at {:?} in space {:?}", node.item, node_info.local_pos, node_info.spatial_node_index);
+                println!("\t\tflags {:?}, resulted in {:?}", node_info.conversion.to_flags(), clip_result);
+            }
+
             match clip_result {
                 ClipResult::Accept => {
                     // Doesn't affect the primitive at all, so skip adding to list
@@ -1325,8 +1338,11 @@ impl ClipItem {
         });
 
         if let Some(inner_clip_rect) = inner_clip_rect {
-            if inner_clip_rect.contains_rect(prim_world_rect) {
-                return ClipResult::Accept;
+            match prim_world_rect.intersection(world_rect) {
+                Some(ref rect) if inner_clip_rect.contains_rect(rect) =>
+                    return ClipResult::Accept,
+                Some(_) => (),
+                None => return ClipResult::Reject,
             }
         }
 

gfx/wr/webrender/src/prim_store/mod.rs

@@ -2051,6 +2051,7 @@ impl PrimitiveStore {
                         &world_culling_rect,
                         &mut frame_state.data_stores.clip,
                         true,
+                        prim_instance.is_chased(),
                     );
 
                 if let Some(ref mut tile_cache) = frame_state.tile_cache {
@@ -3839,6 +3840,7 @@ impl PrimitiveInstance {
                         &dirty_world_rect,
                         &mut data_stores.clip,
                         false,
+                        self.is_chased(),
                     );
 
                 let clip_mask_kind = segment.update_clip_task(