Preserve scroll position during PDF page rendering

Added functionality to maintain the scroll position and viewport
center of the `.pdf-frame` container during PDF page rendering.
This ensures the user's view remains centered on the same content
after re-rendering, even if the canvas dimensions change.

Implemented logic to store the scroll position and viewport center
before rendering and restore them afterward using scaling factors
calculated from the canvas's old and new dimensions.

EnvelopeGenerator.ReceiverUI/wwwroot/js/pdf-viewer.js

@@ -115,6 +115,22 @@ window.pdfViewer = {
         this.pageRendering = true;
 
         try {
+            // Get scroll container
+            const container = this.canvas.closest('.pdf-frame');
+            
+            // Store scroll position and viewport center BEFORE rendering
+            let scrollLeft = 0, scrollTop = 0;
+            let centerX = 0, centerY = 0;
+            let oldWidth = this.canvas.width;
+            let oldHeight = this.canvas.height;
+            
+            if (container) {
+                scrollLeft = container.scrollLeft;
+                scrollTop = container.scrollTop;
+                centerX = scrollLeft + container.clientWidth / 2;
+                centerY = scrollTop + container.clientHeight / 2;
+            }
+
             const page = await this.pdfDoc.getPage(num);
             const viewport = page.getViewport({ scale: this.scale });
 
@@ -138,6 +154,18 @@ window.pdfViewer = {
 
             console.log('Page rendered successfully');
 
+            // Restore viewport center position AFTER rendering
+            if (container && oldWidth > 0 && oldHeight > 0) {
+                const scaleX = this.canvas.width / oldWidth;
+                const scaleY = this.canvas.height / oldHeight;
+                
+                const newCenterX = centerX * scaleX;
+                const newCenterY = centerY * scaleY;
+                
+                container.scrollLeft = newCenterX - container.clientWidth / 2;
+                container.scrollTop = newCenterY - container.clientHeight / 2;
+            }
+
             this.currentRenderTask = null;
             this.pageRendering = false;