Position highlight controls relative to content container offset

Fixed positioning is always relative to the viewport, so set the top
position of the fixed-position highlight controls to take the integration's
content container's vertical offset from the viewport into account.

Translation: ensure that highlight controls are not obscured by PDFjs'
top menu/controls bar.

src/annotator/guest.ts

@@ -242,9 +242,12 @@ export class Guest implements Annotator, Destroyable {
     }
 
     if (this._integration.canStyleClusteredHighlights?.()) {
-      this._clusterToolbar = new HighlightClusterController(element, {
-        features: this.features,
-      });
+      this._clusterToolbar = new HighlightClusterController(
+        this._integration.contentContainer(),
+        {
+          features: this.features,
+        }
+      );
     }
 
     this._hostRPC = new PortRPC();

src/annotator/highlight-clusters.tsx

@@ -77,7 +77,7 @@ export class HighlightClusterController implements Destroyable {
     // For now, the controls are fixed at top-left of screen. This is temporary.
     Object.assign(this._outerContainer.style, {
       position: 'fixed',
-      top: '4px',
+      top: `${this._element.offsetTop + 4}px`,
       left: '4px',
     });