web-platform-tests · mishtiagrawal02-cloud · Dec 27, 2025
diff --git a/00_START_HERE.md b/00_START_HERE.md
diff --git a/COMPLETE_SOLUTION.md b/COMPLETE_SOLUTION.md
diff --git a/DETAILED_ANALYSIS.md b/DETAILED_ANALYSIS.md
@@ -0,0 +1,362 @@
+# Detailed Analysis: Pointer Events with Shadow DOM Slot Removal
+
+## Executive Summary
+
+The WPT test `pointerevent_after_target_removed_from_slot.html` had incorrect assertions about pointer event behavior when DOM elements are removed from shadow DOM slots. This analysis explains:
+
+1. What the issue was
+2. Why it occurred
+3. How it was fixed
+4. The spec-compliant behavior expected
+
+---
+
+## The Core Issue
+
+### Question 1: Should `pointerout`/`pointerleave` be sent to removed elements?
+
+**Issue Description:**
+When a slot element is removed from the shadow tree while a light DOM element (`#filler`) is still hovering over it:
+- The light DOM element (`#filler`) becomes hidden
+- Chrome and Safari dispatched `pointerout` and `pointerleave` to `#filler`
+- Firefox did NOT dispatch these events
+- The test had wrong expectations
+
+**Why This Matters:**
+From a developer perspective, boundary events should come in pairs:
+- `pointerenter` → `pointerleave`
+- `pointerover` → `pointerout`
+
+If only one half of the pair is sent, it can lead to developer bugs where hover states aren't properly cleaned up.
+
+**The Fix:**
+The test was updated to expect `pointerout` and `pointerleave` to be sent:
+```javascript
+"pointerout@filler", "pointerleave@filler",
+```
+
+This is the correct behavior because the pointer is no longer over `#filler` (it's now over the newly-revealed `#parent`).
+
+---
+
+### Question 2: Should `pointerover` be retargeted across shadow boundaries?
+
+**Issue Description:**
+When the pointer is over `#parent` in the shadow DOM:
+- All browsers send a `pointerover` event to `#parent`
+- Chrome and Safari ALSO send a retargeted `pointerover` to `#host` (the shadow root)
+- Firefox does NOT retarget the event to `#host`
+
+**Why This Matters:**
+Event retargeting is a fundamental part of the Shadow DOM spec:
+- Events that originate in shadow DOM should be retargeted when they bubble
+- This prevents light DOM code from seeing internal shadow DOM details
+- However, the event SHOULD reach the shadow host with the proper target
+
+**The Fix:**
+The test was updated to expect the retargeted events:
+```javascript
+"pointerover@parent", "pointerover@host",
+```
+
+This follows the standard Shadow DOM event retargeting pattern.
+
+---
+
+## Technical Deep Dive
+
+### Event Retargeting Rules (from DOM Spec)
+
+When an event bubbles out of a shadow root:
+
+1. The event's `target` is adjusted to be the shadow host
+2. The event's `relatedTarget` (if any) is adjusted
+3. Event listeners on the shadow host see the retargeted event
+
+**Critically**: The `relatedTarget` must NOT expose shadow DOM internals to light DOM code.
+
+---
+
+### The Scenario: Step-by-Step
+
+#### Initial State
+```
+Light DOM:                Shadow DOM:
+<host>                    #parent
+  <filler>    ─────→        <slot>
+                              (filler is placed here)
+```
+
+Pointer is over `#filler`. Events dispatched:
+```
+pointerenter@host
+pointerenter@parent  (in shadow DOM, so retargeted to host)
+pointerenter@slot
+pointerenter@filler
+```
+
+#### Slot Removal (on pointerdown event)
+```
+Light DOM:                Shadow DOM:
+<host>                    #parent
+  <filler>    ───X→        <slot>  ← REMOVED
+  (hidden)
+```
+
+What happens:
+1. `#filler` becomes hidden (no longer in a slot, so not rendered)
+2. Pointer is now over `#parent`
+3. Browser fires boundary events for the change
+
+Expected event sequence:
+```javascript
+"pointerdown@filler",        // The event that triggered removal
+"(removed)",                 // Marker indicating slot was removed
+
+// Now handle the pointer position change
+"pointerout@filler",         // Pointer left filler
+"pointerleave@filler",       // And left filler's ancestry
+"pointerover@parent",        // Pointer now over parent
+"pointerover@host",          // (retargeted from parent)
+"pointerup@parent",          // Subsequent pointer events
+"pointerup@host"
+```
+
+---
+
+## The Three Test Cases
+
+The fixed test covers three different removal scenarios:
+
+### Test 1: `remove-slot`
+Removes the `<slot>` element from shadow DOM
+
+**Before removal:**
+- `#filler` is slotted in `#slot`
+- Pointer over `#filler`
+
+**After removal:**
+- `#filler` is hidden (not in document flow)
+- Pointer over `#parent` (where `#filler` was rendered)
+
+**Expected events after removal:**
+```javascript
+"pointerout@filler",
+"pointerleave@filler",
+"pointerover@parent",
+"pointerover@host",  // retargeted
+"pointerup@parent",
+"pointerup@host"
+```
+
+---
+
+### Test 2: `remove-filler`
+Removes the `#filler` element from light DOM
+
+**Before removal:**
+- `#filler` is slotted in `#slot`
+- Pointer over `#filler`
+
+**After removal:**
+- The slot's fallback content is revealed (empty `<div>`)
+- Pointer over that fallback div
+
+**Expected events after removal:**
+```javascript
+"pointerover@slot",          // Fallback content now revealed
+"pointerup@slot",            // Subsequent events go to fallback
+"pointerdown@slot",
+"pointerup@slot",
+"pointerout@slot",
+"pointerleave@slot",
+"pointerleave@parent",       // Leaving parent
+"pointerleave@host"          // Leaving host
+```
+
+Note: No `pointerout@filler` because `#filler` was removed entirely.
+
+---
+
+### Test 3: `change-slotname`
+Changes the `slot` attribute of `#filler`, unslotting it
+
+**Before removal:**
+- `#filler` has no slot attribute (or slot="")
+- `#filler` is in default slot
+- Pointer over `#filler`
+
+**After removal:**
+- `#filler` has slot="xyz" (non-existent slot)
+- `#filler` is hidden (not slotted)
+- Pointer over `#parent`
+
+**Expected events after removal:**
+```javascript
+"pointerout@filler",         // Similar to remove-slot
+"pointerleave@filler",
+"pointerover@parent",
+"pointerover@host",
+"pointerup@parent",
+"pointerup@host",
+"pointerdown@parent",
+"pointerdown@host",
+"pointerup@parent",
+"pointerup@host",
+"pointerout@parent",
+"pointerout@host",
+"pointerleave@parent",
+"pointerleave@host"
+```
+
+---
+
+## Why Firefox's Behavior Was Different
+
+Firefox's implementation didn't properly handle:
+
+1. **Boundary events during dynamic shadow DOM changes**: When the DOM is modified during pointer operations, Firefox didn't always fire the expected `pointerout`/`pointerleave` events.
+
+2. **Event retargeting**: Firefox may have been skipping retargeting when the original target was no longer in the tree.
+
+The fix in the test expectations validates the correct behavior that Chrome and Safari were already doing.
+
+---
+
+## Implementation Insights
+
+### Key Code Changes
+
+**Original test structure:**
+```javascript
+customElements.define("my-elem", class extends HTMLElement {
+  constructor() {
+    super();
+    let content = document.getElementById("template").content;
+    const shadowRoot = this.attachShadow({ mode: "open" });
+    shadowRoot.appendChild(content.cloneNode(true));
+  }
+});
+```
+
+**Fixed test structure:**
+```html
+<div id="host">
+  <template id="template" shadowrootmode="open">
+    <!-- Shadow DOM content -->
+  </template>
+  <div id="filler"></div>
+</div>
+```
+
+**Why this change matters:**
+1. Declarative shadow roots are more standard and predictable
+2. Eliminates custom element constructor timing issues
+3. Makes test expectations clearer
+
+---
+
+### Event Listener Setup
+
+The test listens for all pointer events at AT_TARGET phase:
+
+```javascript
+const events = [
+  "pointerover", "pointerout",
+  "pointerenter", "pointerleave", 
+  "pointerdown", "pointerup"
+];
+let targets = [host, parent, slot, filler];
+
+for (let i = 0; i < targets.length; i++) {
+  events.forEach(event => targets[i].addEventListener(event, logEvent));
+}
+```
+
+The `logEvent` function only logs AT_TARGET phase events:
+```javascript
+function logEvent(e) {
+  if (e.eventPhase == e.AT_TARGET) {
+    event_log.push(e.type + "@" + e.target.id);
+  }
+}
+```
+
+This ensures we're testing what each element actually receives, not bubble/capture phase artifacts.
+
+---
+
+## Browser Behavioral Patterns
+
+### Chrome/Safari (Correct Behavior)
+✅ Fires boundary events when pointer's position relative to elements changes
+✅ Properly retargets shadow DOM events
+✅ Maintains correct relatedTarget values
+
+### Firefox (Before Fix)
+❌ Doesn't fire boundary events for hidden elements
+❌ Doesn't properly retarget shadow DOM events
+(These are implementation bugs that Firefox would need to fix)
+
+---
+
+## Spec References
+
+### 1. W3C Pointer Events Specification
+- https://w3c.github.io/pointerevents/#firing-events-using-the-pointerevent-interface
+- Defines how pointer events should be fired
+- References DOM event dispatch algorithm
+
+### 2. WHATWG DOM Specification
+- https://dom.spec.whatwg.org/#retarget
+- Defines event retargeting rules for shadow DOM
+- Critical for understanding cross-boundary event behavior
+
+### 3. HTML Living Standard
+- Shadow roots and slots behavior
+- Hit-testing and pointer positioning
+
+---
+
+## Summary of Spec-Compliant Behavior
+
+### Pointer Events in Shadow DOM:
+1. ✅ Events originating in shadow DOM are retargeted when bubbling
+2. ✅ `target` reflects the retargeted element (shadow host when bubbling out)
+3. ✅ Boundary events fire when pointer moves between elements
+4. ✅ `relatedTarget` is adjusted to avoid exposing shadow internals
+5. ✅ Hit-testing respects the flattened DOM tree
+
+### The Fix Validates:
+- Correct event firing order
+- Proper retargeting across shadow boundaries  
+- Boundary event completeness (enter/leave pairs)
+- Privacy (no shadow DOM leakage via relatedTarget)
+
+---
+
+## Chromium Implementation Details
+
+The fix was implemented in Chromium as part of:
+- **Bug**: 404479707
+- **CL**: https://chromium-review.googlesource.com/c/chromium/src/+/7107458
+- **Reviewed by**: Robert Flack
+- **Date**: November 6, 2025
+
+This Chromium fix likely involved:
+1. Proper hit-testing when DOM elements are hidden
+2. Correct event retargeting implementation
+3. Ensuring boundary events are fired for hidden elements
+
+---
+
+## Conclusion
+
+The fix to `pointerevent_after_target_removed_from_slot.html` ensures that:
+
+1. **Test assertions are spec-compliant**: The expected events match the W3C specifications
+2. **All browsers align**: The test validates the correct behavior that Chrome/Safari were already doing
+3. **Boundary events are complete**: Pairing of enter/leave and over/out is maintained
+4. **Shadow DOM privacy is protected**: relatedTarget doesn't leak shadow elements
+
+This is a critical test for ensuring consistent pointer event handling across browsers when working with Shadow DOM slots.