WebRender Clipping Analysis Report

Executive Summary

Problem: Content inside a scroll container is not being visually clipped, even though the clip infrastructure appears to be set up correctly.

Key Finding: The debug output shows that:

1. Clips ARE being defined correctly in WebRender
2. The local_clip_rect is being set properly
3. The clip chain is being built with proper parent relationships
4. BUT: The primitives are not being rejected or masked by the clips

Current Debug Output Analysis

Clip Definition (Correct ✅)

[WR SCENE] RectClip: id=ClipId(1), spatial_id=SpatialId(1), clip_rect=Box2D((68,68), (836,636))
[WR SCENE] ClipChain: id=ClipChainId(0), parent=None
[WR SCENE] RectClip: id=ClipId(2), spatial_id=SpatialId(2), clip_rect=Box2D((0,0), (768,568))
[WR SCENE] ClipChain: id=ClipChainId(1), parent=Some(ClipChainId(0))

Primitive Placement (Correct ✅)

[WR SCENE] Rectangle: bounds=Box2D((20,1640), (780,1800)), clip_chain_id=ClipChainId(1), spatial_id=SpatialId(2)

• Rectangle at y=1640-1800 is outside the clip rect (0,0)-(768,568)
• It should be clipped!

Clip Chain Building (Suspicious ⚠️)

[WR CLIP] build_clip_chain_instance: local_prim_rect=Box2D((20,1640), (780,1800)), local_clip_rect=Box2D((20,1640), (780,1800))
[WR CLIP] local_bounding_rect (intersection)=Box2D((20,1640), (780,1800))

PROBLEM IDENTIFIED: The local_clip_rect equals the local_prim_rect!

This means the clip is NOT being applied. The clip rect should be (0,0)-(768,568) but it's (20,1640)-(780,1800).

Root Cause Analysis

ClipTree / ClipLeaf System

The local_clip_rect comes from clip_leaf.local_clip_rect in set_active_clips():

pub fn set_active_clips(...) {
    let clip_leaf = clip_tree.get_leaf(clip_leaf_id);
    let mut local_clip_rect = clip_leaf.local_clip_rect;  // <-- THIS IS THE PROBLEM
    ...
}

The clip_leaf.local_clip_rect is initialized with the primitive's own clip_rect from CommonItemProperties, NOT from the parent clip chain!

Where the ClipLeaf is Created

In scene_building.rs, when processing DisplayItem::Rectangle:

let current_clip_chain_id = self.get_clip_chain(item.clip_chain_id());
...
let prim_info = CommonItemProperties {
    clip_rect: info.clip_rect,  // <-- This is the primitive's own clip_rect
    clip_chain_id: current_clip_chain_id,
    ...
};

The info.clip_rect is set to the primitive's bounds in our compositor code:

let info = CommonItemProperties {
    clip_rect: rect,  // <-- We set clip_rect = rect (the full primitive bounds)
    clip_chain_id: current_clip_chain_id,
    ...
};

The Fix

The issue is that CommonItemProperties.clip_rect should be the INTERSECTION of the primitive bounds with the active clip region, not just the primitive bounds.

Option 1: Fix in compositor2.rs (Recommended)

For items inside a scroll frame, the clip_rect in CommonItemProperties should be clipped to the scroll frame's viewport:

// Instead of:
let info = CommonItemProperties {
    clip_rect: rect,  // Full primitive rect
    ...
};

// Should be:
let clipped_rect = rect.intersection(&current_clip_bounds).unwrap_or(LayoutRect::zero());
let info = CommonItemProperties {
    clip_rect: clipped_rect,  // Primitive rect clipped to viewport
    ...
};

Option 2: Let WebRender Handle It

WebRender's clip system should handle this through ClipChainId, but the clip_rect field is used as an early-out optimization. If clip_rect is larger than the actual clip region, WebRender may skip clipping.

WebRender Clipping Pipeline

1. Scene Building (scene_building.rs)

• Parses display list items
• Creates clip nodes via define_clip_rect()
• Creates clip chains via define_clip_chain()
• Associates primitives with clip chains

2. Clip Store (clip.rs)

• set_active_clips() - Walks the clip tree to build active clip list
• build_clip_chain_instance() - Creates optimized clip chain for rendering
• get_clip_result() - Determines Accept/Reject/Partial for each clip

3. Visibility (visibility.rs)

• Calls build_clip_chain_instance() for each primitive
• Rejects primitives that are completely clipped
• Creates clip masks for partially clipped primitives

4. GPU Rendering

• Clip masks are texture-based alpha masks
• Simple rect clips can use vertex shader clipping
• Complex clips require mask rendering

Key Data Structures

ClipTree
├── ClipNodeId (0) - Root
│   └── ClipNodeId (1)
│       ├── handle: ClipDataHandle -> ClipNode with clip_rect
│       └── parent: ClipNodeId (0)
│
└── ClipLeafId (0)
    ├── node_id: ClipNodeId (1)
    └── local_clip_rect: LayoutRect  <-- THE PROBLEM SOURCE

Debug Trace Points

1. Scene Building (scene_building.rs):

   • [WR SCENE] RectClip: - When a clip rect is defined
   • [WR SCENE] ClipChain: - When a clip chain is defined
   • [WR SCENE] Rectangle: - When a rectangle primitive is added

2. Clip Store (clip.rs):

   • [WR SET_ACTIVE_CLIPS] - When clips are activated for a primitive
   • [WR CLIP CHAIN] - When a clip node is added to the chain
   • [WR CLIP] build_clip_chain_instance: - When building the final clip instance
   • [WR CLIP] REJECTED: - When a primitive is fully clipped

3. Compositor (compositor2.rs):

   • [CLIP DEBUG] Rect: - Raw and adjusted coordinates
   • [CLIP DEBUG] PushScrollFrame: - Scroll frame definition

Next Steps

1. Add logging to set_active_clips() to see the clip tree traversal
2. Check clip_leaf.local_clip_rect - This might be set incorrectly
3. Trace CommonItemProperties.clip_rect - Ensure it's being intersected with clip bounds
4. Check spatial node coordinate transforms - The clip might be in wrong coordinate space

Files to Investigate

1. /webrender/core/src/clip.rs - ClipStore, set_active_clips, build_clip_chain_instance
2. /webrender/core/src/scene_building.rs - How clips are defined and associated
3. /webrender/core/src/visibility.rs - How primitives are culled
4. /dll/src/desktop/compositor2.rs - How we build the display list

Hypothesis

The ClipLeafId assigned to each primitive determines which clip chain applies. The issue is that:

1. We define a scroll frame clip at (0,0)-(768,568) in scroll space
2. We push rectangles with clip_chain_id pointing to this clip
3. BUT the ClipLeaf.local_clip_rect is set to the primitive's own bounds
4. WebRender uses local_clip_rect as the primary clip source
5. The clip chain clips are only used for masking, not rejection

The fix should ensure that the clip_rect in CommonItemProperties is properly constrained to the scroll frame viewport.