fix(ui): anchor container chrome to the container, not a descendant

`BlockPopover` searched for any `[data-node-type]` under the node view's
DOM, which matches a nested child container's root when the author's own
root isn't stamped - anchoring the popover to a child. The search is now
scoped to the block's own type, and falls back to the container's node
element rather than descending into its contentDOM (the first child's box).

`SideMenu` read `container.firstElementChild!` for the handle's x
position, which is null for a container rendering no element children.

Author: nperez0111

packages/core/src/extensions/SideMenu/SideMenu.ts

@@ -304,7 +304,8 @@ export class SideMenuView<
               // non-nested and we only take the x coordinate, it's ok.
               (
                 getDirectChildBlocks(container, containerUIInfo)[0] ??
-                container.firstElementChild!
+                container.firstElementChild ??
+                container
               ).getBoundingClientRect().x
             : (
                 this.pmView.dom.firstChild as HTMLElement

packages/react/src/components/Popovers/BlockPopover.tsx

@@ -42,12 +42,17 @@ export const BlockPopover = (
           // stamps with `data-node-type`; vanilla containers render that boxed
           // element directly as the node view's DOM.
           if (dom instanceof Element) {
-            const boxed = dom.matches("[data-node-type]")
+            // Scoped to this block's own type: an unscoped descendant search
+            // would match a nested child container's root when the author's
+            // root hasn't been stamped, anchoring the popover to a child.
+            const selector = `[data-node-type="${nodePosInfo.node.type.name}"]`;
+            const boxed = dom.matches(selector)
               ? dom
-              : dom.querySelector("[data-node-type]");
-            if (boxed) {
-              return { element: boxed };
-            }
+              : dom.querySelector(selector);
+            // No stamped root to anchor to, so fall back to the container's
+            // own node element rather than descending into its contentDOM,
+            // which is the first child's box.
+            return { element: boxed ?? dom };
           }
         }