Extend body mutation observer

This adds missing mutation observer unregistration and extends it to
monitor host element resizes too.

service.js

@@ -66,8 +66,12 @@ class BesService {
     this.resizeObserver = new ResizeObserver(this.onResize.bind(this))
     this.resizeObserver.observe(this.hostElement)
 
-    this.initialPosRect = this.hostElement.getBoundingClientRect()
-    this.initializePositionObserver()
+    this.hostBoundingClientRect = this.hostElement.getBoundingClientRect()
+    this.positionObserver = new MutationObserver(this.onBodyMutate.bind(this))
+    this.positionObserver.observe(document.body, {
+      childList: true,
+      subtree: true
+    })
 
     besServices.push(this)
   }
@@ -101,6 +105,7 @@ class BesService {
   unregister() {
     if (this.abortController) this.abortController.abort()
     besServices = besServices.filter(item => item !== this)
+    this.positionObserver.disconnect()
     this.resizeObserver.disconnect()
     this.hostElement.removeEventListener('scroll', this.onScroll)
     this.hostElement.setAttribute('spellcheck', this.originalSpellcheck)
@@ -239,6 +244,24 @@ class BesService {
     this.repositionAllMarkup()
   }
 
+  /**
+   * Called to report document body change
+   */
+  onBodyMutate() {
+    const rect = this.hostElement.getBoundingClientRect()
+    if (
+      rect.top !== this.hostBoundingClientRect.top ||
+      rect.left !== this.hostBoundingClientRect.left
+    )
+      this.onReposition()
+    if (
+      rect.width !== this.hostBoundingClientRect.width ||
+      rect.height !== this.hostBoundingClientRect.height
+    )
+      this.onResize()
+    this.hostBoundingClientRect = rect
+  }
+
   /**
    * Creates grammar mistake markup in DOM.
    *
@@ -475,26 +498,6 @@ class BesService {
         return false
     }
   }
-
-  /**
-   * Initializes position observer. This is necessary to monitor host element movements in DOM.
-   */
-  initializePositionObserver() {
-    const positionObserver = new MutationObserver(() => {
-      const currentPosRect = this.hostElement.getBoundingClientRect()
-      if (
-        currentPosRect.top !== this.initialPosRect.top ||
-        currentPosRect.left !== this.initialPosRect.left
-      ) {
-        this.onReposition()
-        this.initialPosRect = currentPosRect
-      }
-    })
-    positionObserver.observe(document.body, {
-      childList: true,
-      subtree: true
-    })
-  }
 }
 
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