Avoid to compute the client rect of the viewer

The container position and dimensions should be almost constant, hence
it's pretty useless to query them on each rescale.
Finally it avoids to trigger some reflows.

web/app.js

@@ -2363,7 +2363,6 @@ function webViewerResize() {
     // Work-around issue 15324 by ignoring "resize" events during printing.
     return;
   }
-  pdfViewer.updateContainerHeightCss();
 
   if (!pdfDocument) {
     return;
@@ -2646,9 +2645,9 @@ function webViewerWheel(evt) {
       // left corner is restored. When the mouse wheel is used, the position
       // under the cursor should be restored instead.
       const scaleCorrectionFactor = currentScale / previousScale - 1;
-      const rect = pdfViewer.container.getBoundingClientRect();
-      const dx = evt.clientX - rect.left;
-      const dy = evt.clientY - rect.top;
+      const [top, left] = pdfViewer.containerTopLeft;
+      const dx = evt.clientX - left;
+      const dy = evt.clientY - top;
       pdfViewer.container.scrollLeft += dx * scaleCorrectionFactor;
       pdfViewer.container.scrollTop += dy * scaleCorrectionFactor;
     }

web/pdf_viewer.js

@@ -227,10 +227,14 @@ class PDFViewer {
 
   #annotationMode = AnnotationMode.ENABLE_FORMS;
 
+  #containerTopLeft = null;
+
   #enablePermissions = false;
 
   #previousContainerHeight = 0;
 
+  #resizeObserver = new ResizeObserver(this.#resizeObserverCallback.bind(this));
+
   #scrollModePageState = null;
 
   #onVisibilityChange = null;
@@ -247,6 +251,8 @@ class PDFViewer {
       );
     }
     this.container = options.container;
+    this.#resizeObserver.observe(this.container);
+
     this.viewer = options.viewer || options.container.firstElementChild;
 
     if (
@@ -330,7 +336,8 @@ class PDFViewer {
     if (this.removePageBorders) {
       this.viewer.classList.add("removePageBorders");
     }
-    this.updateContainerHeightCss();
+
+    this.#updateContainerHeightCss();
   }
 
   get pagesCount() {
@@ -1137,7 +1144,6 @@ class PDFViewer {
     if (this.defaultRenderingQueue) {
       this.update();
     }
-    this.updateContainerHeightCss();
   }
 
   /**
@@ -2186,16 +2192,32 @@ class PDFViewer {
     this.currentScaleValue = newScale;
   }
 
-  updateContainerHeightCss() {
-    const height = this.container.clientHeight;
-
+  #updateContainerHeightCss(height = this.container.clientHeight) {
     if (height !== this.#previousContainerHeight) {
       this.#previousContainerHeight = height;
-
       docStyle.setProperty("--viewer-container-height", `${height}px`);
     }
   }
 
+  #resizeObserverCallback(entries) {
+    for (const entry of entries) {
+      if (entry.target === this.container) {
+        this.#updateContainerHeightCss(
+          Math.floor(entry.borderBoxSize[0].blockSize)
+        );
+        this.#containerTopLeft = null;
+        break;
+      }
+    }
+  }
+
+  get containerTopLeft() {
+    return (this.#containerTopLeft ||= [
+      this.container.offsetTop,
+      this.container.offsetLeft,
+    ]);
+  }
+
   /**
    * @type {number}
    */