diff --git a/src/browser/Frame.zig b/src/browser/Frame.zig
index afa97ae91..81a380575 100644
--- a/src/browser/Frame.zig
+++ b/src/browser/Frame.zig
@@ -2497,6 +2497,12 @@ pub fn adoptNodeTree(self: *Frame, node: *Node, old_owner: *Document, new_owner:
     // Per spec, adopted steps run on each element after its document is set.
     if (node.is(Element)) |el| {
         Element.Html.Custom.enqueueAdoptedCallbackOnElement(el, old_owner, new_owner, self);
+
+        // The shadow tree follows its host across documents: re-own it and
+        // run its adopted reactions too (spec: shadow-including descendants).
+        if (el.hostedShadowRoot(self)) |shadow_root| {
+            try self.adoptNodeTree(shadow_root.asNode(), old_owner, new_owner);
+        }
     }
 
     var it = node.childrenIterator();
@@ -2632,6 +2638,9 @@ pub fn removeNode(self: *Frame, parent: *Node, child: *Node, opts: RemoveNodeOpt
         }
 
         Element.Html.Custom.enqueueDisconnectedCallbackOnElement(el, self);
+        Element.Html.Custom.enqueueShadowTreeCallbacks(el, .disconnected, self) catch |err| {
+            log.warn(.bug, "ce_reactions enqueue fail", .{ .err = err });
+        };
 
         popover.removeFromOpen(el, self);
 
@@ -2704,19 +2713,31 @@ pub fn moveAllChildren(self: *Frame, source: *Node, parent: *Node, ref_node: ?*N
     // Every child shares source's root, and source itself doesn't move.
     const previous_root = source.getRootNode(.{});
 
+    // Fragment insertion adopts like single-node insertion does. Every child
+    // shares source's owner document, so one comparison covers them all.
+    const source_owner = source.ownerDocument(self);
+    const parent_owner = parent.ownerDocument(self) orelse parent.as(Document);
+    const adopting = source_owner != null and source_owner.? != parent_owner;
+
     var it = source.childrenIterator();
     while (it.next()) |child| {
         try moved.append(self.call_arena, child);
-        self.removeNode(source, child, .{ .reconnect_to = parent, .notify_observers = false });
+        self.removeNode(source, child, .{
+            .reconnect_to = if (adopting) null else parent,
+            .notify_observers = false,
+        });
+        if (adopting) {
+            try self.adoptNodeTree(child, source_owner.?, parent_owner);
+        }
         if (ref_node) |ref| {
             try self.insertNodeRelative(
                 parent,
                 child,
                 .{ .before = ref },
-                .{ .previous_root = previous_root, .notify_observers = false, .run_ready = false },
+                .{ .previous_root = previous_root, .adopting_to_new_document = adopting, .notify_observers = false, .run_ready = false },
             );
         } else {
-            try self.appendNode(parent, child, .{ .previous_root = previous_root, .notify_observers = false, .run_ready = false });
+            try self.appendNode(parent, child, .{ .previous_root = previous_root, .adopting_to_new_document = adopting, .notify_observers = false, .run_ready = false });
         }
     }
 
@@ -2906,6 +2927,7 @@ pub fn _insertNodeRelative(self: *Frame, comptime from_parser: bool, parent: *No
 
         if (should_invoke_connected) {
             try Element.Html.Custom.enqueueConnectedCallbackOnElement(false, el, self);
+            try Element.Html.Custom.enqueueShadowTreeCallbacks(el, .connected, self);
         }
     }
 }
@@ -2977,11 +2999,15 @@ pub fn signalSlotChange(self: *Frame, slot: *Element.Html.Slot) void {
 }
 
 pub fn getCustomizedBuiltInDefinition(self: *Frame, element: *Element) ?*CustomElementDefinition {
+    if (!element._flags.customized_builtin) {
+        return null;
+    }
     return self._customized_builtin_definitions.get(element);
 }
 
 pub fn setCustomizedBuiltInDefinition(self: *Frame, element: *Element, definition: *CustomElementDefinition) !void {
     try self._customized_builtin_definitions.put(self.arena, element, definition);
+    element._flags.customized_builtin = true;
 }
 
 // --- Live range update methods (DOM spec §4.2.3, §4.2.4, §4.7, §4.8) ---
diff --git a/src/browser/dump.zig b/src/browser/dump.zig
index c1b68af57..f9761f3c5 100644
--- a/src/browser/dump.zig
+++ b/src/browser/dump.zig
@@ -138,7 +138,7 @@ fn _deep(node: *Node, opts: Opts, comptime force_slot: bool, writer: *std.Io.Wri
             switch (opts.shadow) {
                 .skip => {},
                 .complete, .rendered => {
-                    if (frame._element_shadow_roots.get(el)) |shadow| {
+                    if (el.hostedShadowRoot(frame)) |shadow| {
                         try children(shadow.asNode(), opts, writer, frame);
                         // In rendered mode, light DOM is only shown through slots, not directly
                         if (opts.shadow == .rendered) {
@@ -153,7 +153,7 @@ fn _deep(node: *Node, opts: Opts, comptime force_slot: bool, writer: *std.Io.Wri
                     }
                 },
                 .declarative => |declarative| {
-                    if (frame._element_shadow_roots.get(el)) |shadow| {
+                    if (el.hostedShadowRoot(frame)) |shadow| {
                         if (shouldSerializeShadow(shadow, declarative)) {
                             try writeDeclarativeShadow(shadow, opts, writer, frame);
                         }
@@ -223,7 +223,7 @@ fn _deep(node: *Node, opts: Opts, comptime force_slot: bool, writer: *std.Io.Wri
 pub fn getHTML(node: *Node, declarative: Opts.Shadow.Declarative, writer: *std.Io.Writer, frame: *Frame) !void {
     const opts = Opts{ .shadow = .{ .declarative = declarative } };
     if (node.is(Node.Element)) |el| {
-        if (frame._element_shadow_roots.get(el)) |shadow| {
+        if (el.hostedShadowRoot(frame)) |shadow| {
             if (shouldSerializeShadow(shadow, declarative)) {
                 // if the element's shadowroot tree is rendered before its
                 // children (assume the opts say that it should serialize the
diff --git a/src/browser/frame/parse.zig b/src/browser/frame/parse.zig
index 5ff8b9ba2..ea00bdece 100644
--- a/src/browser/frame/parse.zig
+++ b/src/browser/frame/parse.zig
@@ -59,7 +59,7 @@ fn htmlAsChildrenInner(frame: *Frame, node: *Node, html: []const u8, opts: Fragm
             slotting.assignSlottablesForTree(root, frame);
         }
         if (node.is(Element)) |el| {
-            if (frame._element_shadow_roots.get(el)) |shadow_root| {
+            if (el.hostedShadowRoot(frame)) |shadow_root| {
                 slotting.assignSlottablesForTree(shadow_root.asNode(), frame);
             }
         }
diff --git a/src/browser/markdown.zig b/src/browser/markdown.zig
index 3606a4d6a..3a68e41db 100644
--- a/src/browser/markdown.zig
+++ b/src/browser/markdown.zig
@@ -439,7 +439,7 @@ const Context = struct {
         // path (cf. dump.zig's default .rendered mode), so we always pierce; the
         // early-return tags above can never be valid shadow hosts, so only this
         // generic path needs the check.
-        if (self.frame._element_shadow_roots.get(el)) |shadow| {
+        if (el.hostedShadowRoot(self.frame)) |shadow| {
             try self.renderChildren(shadow.asNode());
         } else {
             try self.renderChildren(el.asNode());
diff --git a/src/browser/tests/custom_elements/shadow_tree_callbacks.html b/src/browser/tests/custom_elements/shadow_tree_callbacks.html
new file mode 100644
index 000000000..a00b5ea6c
--- /dev/null
+++ b/src/browser/tests/custom_elements/shadow_tree_callbacks.html
@@ -0,0 +1,84 @@
+<!DOCTYPE html>
+<body>
+<script src="../testing.js"></script>
+<script id="shadow_tree_callbacks">
+{
+    let calls = [];
+
+    class MyElement extends HTMLElement {
+        connectedCallback() { calls.push('connected'); }
+        adoptedCallback(oldDoc, newDoc) {
+            calls.push('adopted');
+            calls.push(oldDoc === document);
+            calls.push(newDoc === otherDoc);
+        }
+        disconnectedCallback() { calls.push('disconnected'); }
+    }
+    customElements.define('my-shadowed-element', MyElement);
+
+    const otherDoc = document.implementation.createHTMLDocument('other');
+
+    function makeHost(mode) {
+        const host = document.createElement('div');
+        const shadow = host.attachShadow({mode: mode});
+        shadow.appendChild(document.createElement('my-shadowed-element'));
+        return host;
+    }
+
+    // Case 1: inserting a host fires connectedCallback inside its shadow tree.
+    {
+        const host = makeHost('open');
+        calls = [];
+        document.body.appendChild(host);
+        testing.expectEqual('connected', calls.join(','));
+
+        // and removing it fires disconnectedCallback.
+        calls = [];
+        host.remove();
+        testing.expectEqual('disconnected', calls.join(','));
+    }
+
+    // Case 2: closed shadow trees get the callbacks too.
+    {
+        const host = makeHost('closed');
+        calls = [];
+        document.body.appendChild(host);
+        host.remove();
+        testing.expectEqual('connected,disconnected', calls.join(','));
+    }
+
+    // Case 3: nested shadow trees (a host inside another host's shadow tree).
+    {
+        const outer = document.createElement('div');
+        outer.attachShadow({mode: 'open'}).appendChild(makeHost('open'));
+        calls = [];
+        document.body.appendChild(outer);
+        outer.remove();
+        testing.expectEqual('connected,disconnected', calls.join(','));
+    }
+
+    // Case 4: adopting a connected host into another document runs the shadow
+    // tree's adopted (and disconnected/connected) reactions, and re-owns it.
+    {
+        const host = makeHost('open');
+        const inner = host.shadowRoot.firstChild;
+        document.body.appendChild(host);
+        calls = [];
+        otherDoc.body.appendChild(host);
+        testing.expectEqual('disconnected,adopted,true,true,connected', calls.join(','));
+        testing.expectEqual(otherDoc, inner.ownerDocument);
+    }
+
+    // Case 5: inserting a shadow root (a DocumentFragment) into another
+    // document moves and adopts its children.
+    {
+        const host = makeHost('open');
+        const shadow = host.shadowRoot;
+        document.body.appendChild(host);
+        calls = [];
+        otherDoc.body.appendChild(shadow);
+        testing.expectEqual('disconnected,adopted,true,true,connected', calls.join(','));
+    }
+}
+</script>
+</body>
diff --git a/src/browser/webapi/Element.zig b/src/browser/webapi/Element.zig
index befc812f4..a11caf0b9 100644
--- a/src/browser/webapi/Element.zig
+++ b/src/browser/webapi/Element.zig
@@ -132,8 +132,19 @@ pub const Namespace = enum(u8) {
     }
 };
 
+pub const Flags = packed struct(u8) {
+    shadow_host: bool = false,
+    customized_builtin: bool = false,
+    _unused: u6 = 0,
+};
+
 _type: Type,
 _namespace: Namespace = .html,
+// Presence hints for the frame's element-keyed side tables: a set bit means
+// "maybe in the map" (the map stays the authority), a clear bit skips the
+// lookup. Turns the per-element map probe in tree walks into a bit test on
+// memory the walk already touches. Fits in existing struct padding.
+_flags: Flags = .{},
 _attributes: Attribute.List = .{},
 // In debug, set so that we can check that we have a proper contiguous block
 // of memory for the entire chain (and thus, simple pointer arithmetics will
@@ -624,31 +635,6 @@ pub fn setDir(self: *Element, value: []const u8, frame: *Frame) !void {
     return self.setAttributeSafe(comptime .wrap("dir"), .wrap(value), frame);
 }
 
-// ARIAMixin - ARIA attribute reflection
-pub fn getAriaAtomic(self: *const Element) ?[]const u8 {
-    return self.getAttributeSafe(comptime .wrap("aria-atomic"));
-}
-
-pub fn setAriaAtomic(self: *Element, value: ?[]const u8, frame: *Frame) !void {
-    if (value) |v| {
-        try self.setAttributeSafe(comptime .wrap("aria-atomic"), .wrap(v), frame);
-    } else {
-        try self.removeAttribute(comptime .wrap("aria-atomic"), frame);
-    }
-}
-
-pub fn getAriaLive(self: *const Element) ?[]const u8 {
-    return self.getAttributeSafe(comptime .wrap("aria-live"));
-}
-
-pub fn setAriaLive(self: *Element, value: ?[]const u8, frame: *Frame) !void {
-    if (value) |v| {
-        try self.setAttributeSafe(comptime .wrap("aria-live"), .wrap(v), frame);
-    } else {
-        try self.removeAttribute(comptime .wrap("aria-live"), frame);
-    }
-}
-
 pub fn getClassName(self: *const Element) []const u8 {
     return self.getAttributeSafe(comptime .wrap("class")) orelse "";
 }
@@ -815,7 +801,7 @@ pub fn setAttributeSafe(self: *Element, name: String, value: String, frame: *Fra
 }
 
 pub fn getShadowRoot(self: *Element, frame: *Frame) ?*ShadowRoot {
-    const shadow_root = frame._element_shadow_roots.get(self) orelse return null;
+    const shadow_root = self.hostedShadowRoot(frame) orelse return null;
     if (shadow_root._mode == .closed) return null;
     return shadow_root;
 }
@@ -852,7 +838,7 @@ pub fn attachShadow(self: *Element, opts: ShadowRoot.AttachOptions, frame: *Fram
         }
     }
 
-    if (frame._element_shadow_roots.get(self)) |existing| {
+    if (self.hostedShadowRoot(frame)) |existing| {
         // Imperative attachShadow over a declarative shadow root with a matching
         // mode empties it and returns the same root. The parser
         // (opts.declarative) never replaces an existing root.
@@ -866,9 +852,20 @@ pub fn attachShadow(self: *Element, opts: ShadowRoot.AttachOptions, frame: *Fram
 
     const shadow_root = try ShadowRoot.init(self, opts, frame);
     try frame._element_shadow_roots.put(frame.arena, self, shadow_root);
+    self._flags.shadow_host = true;
     return shadow_root;
 }
 
+// The shadow root this element hosts, closed ones included (the JS-facing
+// getShadowRoot filters those). The flag check skips the map probe for the
+// overwhelming majority of elements, which host nothing.
+pub fn hostedShadowRoot(self: *Element, frame: *const Frame) ?*ShadowRoot {
+    if (!self._flags.shadow_host) {
+        return null;
+    }
+    return frame._element_shadow_roots.get(self);
+}
+
 pub fn insertAdjacentElement(
     self: *Element,
     position: []const u8,
@@ -1789,7 +1786,7 @@ pub fn clone(self: *Element, deep: bool, frame: *Frame) !*Node {
 
     // Per spec, a clonable shadow root is cloned along with its host — its
     // children always deep-cloned, even when the host clone is shallow.
-    if (frame._element_shadow_roots.get(self)) |shadow| {
+    if (self.hostedShadowRoot(frame)) |shadow| {
         if (shadow._clonable) {
             const cloned_shadow = node.as(Element).attachShadow(.{
                 .mode = shadow._mode,
@@ -2362,8 +2359,50 @@ pub const JsApi = struct {
     pub const localName = bridge.accessor(Element.getLocalName, null, .{});
     pub const id = bridge.accessor(Element.getId, Element.setId, .{ .ce_reactions = true });
     pub const slot = bridge.accessor(Element.getSlot, Element.setSlot, .{ .ce_reactions = true });
-    pub const ariaAtomic = bridge.accessor(Element.getAriaAtomic, Element.setAriaAtomic, .{ .ce_reactions = true });
-    pub const ariaLive = bridge.accessor(Element.getAriaLive, Element.setAriaLive, .{ .ce_reactions = true });
+    pub const role = ariaAccessor("role");
+    pub const ariaAtomic = ariaAccessor("aria-atomic");
+    pub const ariaAutoComplete = ariaAccessor("aria-autocomplete");
+    pub const ariaBrailleLabel = ariaAccessor("aria-braillelabel");
+    pub const ariaBrailleRoleDescription = ariaAccessor("aria-brailleroledescription");
+    pub const ariaBusy = ariaAccessor("aria-busy");
+    pub const ariaChecked = ariaAccessor("aria-checked");
+    pub const ariaColCount = ariaAccessor("aria-colcount");
+    pub const ariaColIndex = ariaAccessor("aria-colindex");
+    pub const ariaColIndexText = ariaAccessor("aria-colindextext");
+    pub const ariaColSpan = ariaAccessor("aria-colspan");
+    pub const ariaCurrent = ariaAccessor("aria-current");
+    pub const ariaDescription = ariaAccessor("aria-description");
+    pub const ariaDisabled = ariaAccessor("aria-disabled");
+    pub const ariaExpanded = ariaAccessor("aria-expanded");
+    pub const ariaHasPopup = ariaAccessor("aria-haspopup");
+    pub const ariaHidden = ariaAccessor("aria-hidden");
+    pub const ariaInvalid = ariaAccessor("aria-invalid");
+    pub const ariaKeyShortcuts = ariaAccessor("aria-keyshortcuts");
+    pub const ariaLabel = ariaAccessor("aria-label");
+    pub const ariaLevel = ariaAccessor("aria-level");
+    pub const ariaLive = ariaAccessor("aria-live");
+    pub const ariaModal = ariaAccessor("aria-modal");
+    pub const ariaMultiLine = ariaAccessor("aria-multiline");
+    pub const ariaMultiSelectable = ariaAccessor("aria-multiselectable");
+    pub const ariaOrientation = ariaAccessor("aria-orientation");
+    pub const ariaPlaceholder = ariaAccessor("aria-placeholder");
+    pub const ariaPosInSet = ariaAccessor("aria-posinset");
+    pub const ariaPressed = ariaAccessor("aria-pressed");
+    pub const ariaReadOnly = ariaAccessor("aria-readonly");
+    pub const ariaRelevant = ariaAccessor("aria-relevant");
+    pub const ariaRequired = ariaAccessor("aria-required");
+    pub const ariaRoleDescription = ariaAccessor("aria-roledescription");
+    pub const ariaRowCount = ariaAccessor("aria-rowcount");
+    pub const ariaRowIndex = ariaAccessor("aria-rowindex");
+    pub const ariaRowIndexText = ariaAccessor("aria-rowindextext");
+    pub const ariaRowSpan = ariaAccessor("aria-rowspan");
+    pub const ariaSelected = ariaAccessor("aria-selected");
+    pub const ariaSetSize = ariaAccessor("aria-setsize");
+    pub const ariaSort = ariaAccessor("aria-sort");
+    pub const ariaValueMax = ariaAccessor("aria-valuemax");
+    pub const ariaValueMin = ariaAccessor("aria-valuemin");
+    pub const ariaValueNow = ariaAccessor("aria-valuenow");
+    pub const ariaValueText = ariaAccessor("aria-valuetext");
     pub const dir = bridge.accessor(Element.getDir, Element.setDir, .{ .ce_reactions = true });
     pub const className = bridge.accessor(Element.getClassName, Element.setClassName, .{ .ce_reactions = true });
     pub const classList = bridge.accessor(Element.getClassList, Element.setClassList, .{ .ce_reactions = true });
@@ -2463,6 +2502,23 @@ pub const JsApi = struct {
     pub const scroll = bridge.function(Element.scrollTo, .{});
     pub const scrollTo = bridge.function(Element.scrollTo, .{});
     pub const scrollBy = bridge.function(Element.scrollBy, .{});
+
+    fn ariaAccessor(comptime attr: []const u8) js.bridge.Accessor {
+        const R = struct {
+            pub fn get(self: *const Element) ?[]const u8 {
+                return self.getAttributeSafe(.wrap(attr));
+            }
+
+            pub fn set(self: *Element, value: ?[]const u8, frame: *Frame) !void {
+                if (value) |v| {
+                    try self.setAttributeSafe(.wrap(attr), .wrap(v), frame);
+                } else {
+                    try self.removeAttribute(.wrap(attr), frame);
+                }
+            }
+        };
+        return bridge.accessor(R.get, R.set, .{ .ce_reactions = true });
+    }
 };
 
 pub const Build = struct {
@@ -2502,3 +2558,11 @@ const testing = @import("../../testing.zig");
 test "WebApi: Element" {
     try testing.htmlRunner("element", .{});
 }
+
+test "Element: div chain slot size" {
+    // Guard against accidental growth: new Element fields (e.g. _flags) must
+    // fit in existing padding. Debug is larger from the _proto_canary fields.
+    const Div = @import("element/html/Div.zig");
+    const slot = comptime Factory.chainOffsetOf(Div, Div) + @sizeOf(Div);
+    try testing.expectEqual(if (comptime lp.IS_DEBUG) 120 else 74, slot);
+}
diff --git a/src/browser/webapi/element/html/Custom.zig b/src/browser/webapi/element/html/Custom.zig
index 153657bef..409f8800e 100644
--- a/src/browser/webapi/element/html/Custom.zig
+++ b/src/browser/webapi/element/html/Custom.zig
@@ -25,6 +25,7 @@ const Frame = @import("../../../Frame.zig");
 
 const Node = @import("../../Node.zig");
 const Element = @import("../../Element.zig");
+const TreeWalker = @import("../../TreeWalker.zig");
 const Document = @import("../../Document.zig");
 const HtmlElement = @import("../Html.zig");
 const CustomElementDefinition = @import("../../CustomElementDefinition.zig");
@@ -142,16 +143,40 @@ pub fn enqueueDisconnectedCallbackOnElement(element: *Element, frame: *Frame) vo
 
 // Enqueues an atomic-move reaction (moveBefore). Unlike connect/disconnect there
 // is no dedup state to flip: a move always fires, and the element's connected
-// state is unchanged by the move.
+// state is unchanged by the move. The element's shadow tree (if any) always
+// moves with it.
 pub fn enqueueMoveCallbackOnElement(element: *Element, frame: *Frame) void {
-    if (element.is(Custom)) |custom| {
-        if (custom._definition == null) return;
-    } else {
-        if (frame.getCustomizedBuiltInDefinition(element) == null) return;
+    const eligible = if (element.is(Custom)) |custom|
+        custom._definition != null
+    else
+        frame.getCustomizedBuiltInDefinition(element) != null;
+
+    if (eligible) {
+        frame._ce_reactions.enqueueMove(frame, element) catch |err| {
+            log.warn(.bug, "ce_reactions enqueue fail", .{ .err = err });
+        };
+    }
+
+    const shadow_root = element.hostedShadowRoot(frame) orelse return;
+    var tw = TreeWalker.FullExcludeSelf.Elements.init(shadow_root.asNode(), .{});
+    while (tw.next()) |el| {
+        enqueueMoveCallbackOnElement(el, frame);
+    }
+}
+
+// Reactions descend through the shadodom, so when an element is connected or
+// disconnected, we need to enqueue the connect/disconnect callback for any
+// nested element including those nested in a shadow root.
+pub fn enqueueShadowTreeCallbacks(host: *Element, comptime reaction: enum { connected, disconnected }, frame: *Frame) error{OutOfMemory}!void {
+    const shadow_root = host.hostedShadowRoot(frame) orelse return;
+    var tw = TreeWalker.FullExcludeSelf.Elements.init(shadow_root.asNode(), .{});
+    while (tw.next()) |el| {
+        switch (comptime reaction) {
+            .connected => try enqueueConnectedCallbackOnElement(false, el, frame),
+            .disconnected => enqueueDisconnectedCallbackOnElement(el, frame),
+        }
+        try enqueueShadowTreeCallbacks(el, reaction, frame);
     }
-    frame._ce_reactions.enqueueMove(frame, element) catch |err| {
-        log.warn(.bug, "ce_reactions enqueue fail", .{ .err = err });
-    };
 }
 
 pub fn enqueueAdoptedCallbackOnElement(element: *Element, old_document: *Document, new_document: *Document, frame: *Frame) void {
diff --git a/src/browser/webapi/element/slotting.zig b/src/browser/webapi/element/slotting.zig
index 1e26ca1d6..8acbfcd88 100644
--- a/src/browser/webapi/element/slotting.zig
+++ b/src/browser/webapi/element/slotting.zig
@@ -41,7 +41,7 @@ pub fn isSlottable(node: *Node) bool {
 pub fn findSlot(slottable: *Node, comptime open_only: bool, frame: *Frame) ?*Slot {
     const parent = slottable.parentElement() orelse return null;
 
-    const shadow_root = frame._element_shadow_roots.get(parent) orelse return null;
+    const shadow_root = parent.hostedShadowRoot(frame) orelse return null;
 
     if (open_only and shadow_root._mode != .open) {
         return null;
@@ -172,7 +172,7 @@ fn subtreeHasSlot(node: *Node) bool {
 pub fn insertionSteps(parent: *Node, child: *Node, in_fragment_parse: bool, frame: *Frame) void {
     // The new child may be a slottable to assign in the parent's shadow tree.
     if (parent.is(Element)) |parent_el| {
-        if (frame._element_shadow_roots.get(parent_el) != null and isSlottable(child)) {
+        if (parent_el.hostedShadowRoot(frame) != null and isSlottable(child)) {
             assignASlot(child, frame);
         }
     }