[REF] web: simplify iframe retrieval in position hook

This commit reworks and simplifies the `getIFrame()` method in the
following way : instead of trying to find/guess the targetted element
ancestor's document and then looking in this parent for the iframe
containing the targetted element, it now uses the popperEl as parent for
the iframes who may contain the targetted element.

In practice, the only caveats is that it won't support multiple nested
iframes (which we shouldn't use anyway in our codebase).

Part-of: odoo/odoo#138847

addons/web/static/src/core/position_hook.js

@@ -86,16 +86,13 @@ const DEFAULTS = {
 };
 
 /**
- * @param {HTMLElement} el
+ * @param {HTMLElement} popperEl
+ * @param {HTMLElement} targetEl
  * @returns {HTMLIFrameElement?}
  */
-function getIFrame(el) {
-    const parentDocument = el.ownerDocument.defaultView.parent?.document;
-    if (!parentDocument || parentDocument === el.ownerDocument) {
-        return;
-    }
-    return [...parentDocument.getElementsByTagName("iframe")].find((iframe) =>
-        iframe.contentDocument.contains(el)
+function getIFrame(popperEl, targetEl) {
+    return [...popperEl.ownerDocument.getElementsByTagName("iframe")].find((iframe) =>
+        iframe.contentDocument?.contains(targetEl)
     );
 }
 
@@ -319,7 +316,7 @@ export function usePosition(refName, getTarget, options = {}) {
         }
 
         // Prepare
-        const iframe = getIFrame(targetEl);
+        const iframe = getIFrame(ref.el, targetEl);
         reposition(ref.el, targetEl, { ...DEFAULTS, ...options }, iframe);
     };