Don't re-create the structTreeLayer on zooming and rotation

Compared to the recent PR 15722 for the `textLayer` this one should be a (comparatively) much a smaller win overall, since most documents don't have any structTree-data and the required parsing should be cheaper. However, it seems to me that it cannot hurt to improve this nonetheless.

Note that by moving the `structTreeLayer` initialization we remove the need for the "textlayerrendered" event listener, which thus simplifies the code a little bit.

Also, removes the API-caching of the structTree-data since this was basically done to offset the lack of caching in the viewer.

src/display/api.js

@@ -1623,9 +1623,7 @@ class PDFPageProxy {
    *   or `null` when no structure tree is present for the current page.
    */
   getStructTree() {
-    return (this._structTreePromise ||= this._transport.getStructTree(
-      this._pageIndex
-    ));
+    return this._transport.getStructTree(this._pageIndex);
   }
 
   /**
@@ -1659,7 +1657,6 @@ class PDFPageProxy {
     this._bitmaps.clear();
     this._annotationPromises.clear();
     this._jsActionsPromise = null;
-    this._structTreePromise = null;
     this.pendingCleanup = false;
     return Promise.all(waitOn);
   }
@@ -1694,7 +1691,6 @@ class PDFPageProxy {
     this.objs.clear();
     this._annotationPromises.clear();
     this._jsActionsPromise = null;
-    this._structTreePromise = null;
     if (resetStats && this._stats) {
       this._stats = new StatTimer();
     }

web/pdf_page_view.js

@@ -318,6 +318,33 @@ class PDFPageView {
       numTextDivs: textLayer.numTextDivs,
       error,
     });
+
+    if (this.structTreeLayerFactory) {
+      this.#renderStructTreeLayer();
+    }
+  }
+
+  /**
+   * The structure tree is currently only supported when the text layer is
+   * enabled and a canvas is used for rendering.
+   *
+   * The structure tree must be generated after the text layer for the
+   * aria-owns to work.
+   */
+  async #renderStructTreeLayer() {
+    if (!this.textLayer) {
+      return;
+    }
+    this.structTreeLayer ||=
+      this.structTreeLayerFactory.createStructTreeLayerBuilder();
+
+    const tree = await (!this.structTreeLayer.renderingDone
+      ? this.pdfPage.getStructTree()
+      : null);
+    const treeDom = this.structTreeLayer?.render(tree);
+    if (treeDom) {
+      this.canvas?.append(treeDom);
+    }
   }
 
   async _buildXfaTextContentItems(textDivs) {
@@ -578,6 +605,9 @@ class PDFPageView {
       this.textLayer.cancel();
       this.textLayer = null;
     }
+    if (this.structTreeLayer && !this.textLayer) {
+      this.structTreeLayer = null;
+    }
     if (
       this.annotationLayer &&
       (!keepAnnotationLayer || !this.annotationLayer.div)
@@ -598,10 +628,6 @@ class PDFPageView {
       this.xfaLayer = null;
       this.textHighlighter?.disable();
     }
-    if (this._onTextLayerRendered) {
-      this.eventBus._off("textlayerrendered", this._onTextLayerRendered);
-      this._onTextLayerRendered = null;
-    }
   }
 
   cssTransform({
@@ -844,38 +870,6 @@ class PDFPageView {
       this._renderXfaLayer();
     }
 
-    // The structure tree is currently only supported when the text layer is
-    // enabled and a canvas is used for rendering.
-    if (this.structTreeLayerFactory && this.textLayer && this.canvas) {
-      // The structure tree must be generated after the text layer for the
-      // aria-owns to work.
-      this._onTextLayerRendered = event => {
-        if (event.pageNumber !== this.id) {
-          return;
-        }
-        this.eventBus._off("textlayerrendered", this._onTextLayerRendered);
-        this._onTextLayerRendered = null;
-
-        if (!this.canvas) {
-          return; // The canvas was removed, prevent errors below.
-        }
-        this.pdfPage.getStructTree().then(tree => {
-          if (!tree) {
-            return;
-          }
-          if (!this.canvas) {
-            return; // The canvas was removed, prevent errors below.
-          }
-          const treeDom = this.structTreeLayer.render(tree);
-          treeDom.classList.add("structTree");
-          this.canvas.append(treeDom);
-        });
-      };
-      this.eventBus._on("textlayerrendered", this._onTextLayerRendered);
-      this.structTreeLayer =
-        this.structTreeLayerFactory.createStructTreeLayerBuilder();
-    }
-
     div.setAttribute("data-loaded", true);
 
     this.eventBus.dispatch("pagerender", {

web/struct_tree_layer_builder.js

@@ -72,8 +72,19 @@ const PDF_ROLE_TO_HTML_ROLE = {
 const HEADING_PATTERN = /^H(\d+)$/;
 
 class StructTreeLayerBuilder {
+  #treeDom = undefined;
+
+  get renderingDone() {
+    return this.#treeDom !== undefined;
+  }
+
   render(structTree) {
-    return this.#walk(structTree);
+    if (this.#treeDom !== undefined) {
+      return this.#treeDom;
+    }
+    const treeDom = this.#walk(structTree);
+    treeDom?.classList.add("structTree");
+    return (this.#treeDom = treeDom);
   }
 
   #setAttributes(structElement, htmlElement) {