Improve PDF viewer overlay synchronization

Refactor `EnvelopeSenderEditorPage.razor` to enhance the structure and behavior of the PDF editor wrapper:
- Add `class="pdf-editor-wrapper"` and update `overflow` to `auto`.
- Update `DxPdfViewer`'s `CssClass` to `sender-editor-pdf-viewer`.
- Introduce `OnAfterRenderAsync` to synchronize the overlay with the viewer.

Add new styles in `envelope-viewer.css` for better layout:
- Ensure `.pdf-editor-wrapper` and `.sender-editor-pdf-viewer` occupy full dimensions.
- Center and align content within the PDF viewer.

Enhance `envelope-editor.js` with `syncOverlayToPage`:
- Dynamically adjust overlay position and size relative to the viewer.
- Use `MutationObserver` and event listeners for real-time synchronization.
- Handle delayed rendering with scheduled sync attempts.

These changes improve overlay alignment, user experience, and code maintainability.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Components/Pages/EnvelopeSenderEditorPage.razor

@@ -147,15 +147,15 @@
         else
         {
             @* PDF viewer + overlay wrapper *@
-            <div id="pdf-editor-wrapper"
-                 style="position: relative; width: 100%; height: 100%; overflow: hidden;">
+            <div id="pdf-editor-wrapper" class="pdf-editor-wrapper"
+                 style="position: relative; width: 100%; height: 100%; overflow: auto;">
 
                 @* DxPdfViewer — zoom fixed to 1.0 for reliable coordinate mapping *@
                 <DxPdfViewer @ref="_pdfViewer"
                              DocumentContent="@_pdfBytes"
                              ZoomLevel="1.0"
                              IsSinglePagePreview="false"
-                             CssClass="w-100 h-100" />
+                             CssClass="sender-editor-pdf-viewer" />
 
                 @* Transparent overlay for click capture (active only in placement mode) *@
                 <div id="pdf-editor-overlay"
@@ -336,6 +336,17 @@
             $"[SenderEditor] Total fields: {_signatureFields.Count}");
     }
 
+    protected override async Task OnAfterRenderAsync(bool firstRender)
+    {
+        if (!_pdfLoaded || _errorMessage is not null)
+            return;
+
+        await JSRuntime.InvokeVoidAsync(
+            "envelopeEditor.syncOverlayToPage",
+            "pdf-editor-wrapper",
+            "pdf-editor-overlay");
+    }
+
     // ── Models ──
     record SignatureFieldDraft(double XPt, double YPt, int Page, double DisplayX, double DisplayY);
 

EnvelopeGenerator.Server/EnvelopeGenerator.Server/wwwroot/css/envelope-viewer.css

@@ -51,6 +51,33 @@
     overflow: auto;
 }
 
+.pdf-editor-wrapper {
+    position: relative;
+    min-height: 100%;
+}
+
+.sender-editor-pdf-viewer {
+    width: 100%;
+    height: 100%;
+}
+
+.sender-editor-pdf-viewer .dxbrv-document-surface {
+    display: flex;
+    flex-direction: column;
+    align-items: center;
+}
+
+.sender-editor-pdf-viewer .dxbrv-report-preview-content-flex-item {
+    width: 100%;
+    display: flex;
+    justify-content: center;
+}
+
+.sender-editor-pdf-viewer .dxbrv-report-preview-content {
+    margin-left: auto;
+    margin-right: auto;
+}
+
 .pdf-viewer-container {
     height: 100%;
     display: flex;

EnvelopeGenerator.Server/EnvelopeGenerator.Server/wwwroot/js/envelope-editor.js

@@ -1,4 +1,6 @@
 window.envelopeEditor = {
+    _overlaySyncState: {},
+
     /**
      * Returns click coordinates relative to the overlay element.
      * @param {string} overlayId - The id of the overlay div
@@ -17,5 +19,74 @@ window.envelopeEditor = {
             containerW: rect.width,
             containerH: rect.height
         };
+    },
+
+    syncOverlayToPage: function (wrapperId, overlayId) {
+        const wrapper = document.getElementById(wrapperId);
+        const overlay = document.getElementById(overlayId);
+
+        if (!wrapper || !overlay) {
+            return;
+        }
+
+        const existing = window.envelopeEditor._overlaySyncState[overlayId];
+        if (existing) {
+            return existing.sync();
+        }
+
+        const findTarget = (currentWrapper) => {
+            const page = currentWrapper.querySelector(".dxbrv-report-preview-content");
+            if (page) {
+                return page;
+            }
+
+            return currentWrapper.querySelector(".dxbrv-report-preview-content-img") ||
+                currentWrapper.querySelector("img.dxbrv-report-preview-content-img") ||
+                currentWrapper.querySelector(".dxbrv-document-surface img");
+        };
+
+        const sync = () => {
+            const currentWrapper = document.getElementById(wrapperId);
+            const currentOverlay = document.getElementById(overlayId);
+
+            if (!currentWrapper || !currentOverlay) {
+                return;
+            }
+
+            const target = findTarget(currentWrapper);
+            if (!target) {
+                currentOverlay.style.left = "0px";
+                currentOverlay.style.top = "0px";
+                currentOverlay.style.width = "0px";
+                currentOverlay.style.height = "0px";
+                return;
+            }
+
+            const wrapperRect = currentWrapper.getBoundingClientRect();
+            const targetRect = target.getBoundingClientRect();
+
+            currentOverlay.style.left = `${targetRect.left - wrapperRect.left + currentWrapper.scrollLeft}px`;
+            currentOverlay.style.top = `${targetRect.top - wrapperRect.top + currentWrapper.scrollTop}px`;
+            currentOverlay.style.width = `${targetRect.width}px`;
+            currentOverlay.style.height = `${targetRect.height}px`;
+        };
+
+        const scheduleSync = () => requestAnimationFrame(sync);
+
+        const observer = new MutationObserver(scheduleSync);
+        observer.observe(wrapper, { childList: true, subtree: true, attributes: true });
+
+        wrapper.addEventListener("scroll", scheduleSync, { passive: true });
+        window.addEventListener("resize", scheduleSync);
+
+        window.envelopeEditor._overlaySyncState[overlayId] = {
+            sync,
+            observer
+        };
+
+        sync();
+        setTimeout(sync, 50);
+        setTimeout(sync, 150);
+        setTimeout(sync, 400);
     }
 };