Refactor setScrollMode/setSpreadMode, in the viewer classes, such that they are no-ops in PDFSinglePageViewer instances

Since the Scroll/Spread modes doesn't make (any) sense in `PDFSinglePageViewer` instances, the general structure of these methods can be improved to reflect that.

web/base_viewer.js

@@ -1020,26 +1020,10 @@ class BaseViewer {
   }
 
   setScrollMode(mode) {
-    if (mode === this.scrollMode) {
-      return;
-    }
     if (!Number.isInteger(mode) || !Object.values(ScrollMode).includes(mode)) {
       throw new Error(`Invalid scroll mode: ${mode}`);
     }
-
     this.scrollMode = mode;
-    this._updateScrollModeClasses();
-    this.eventBus.dispatch('scrollmodechanged', { mode, });
-
-    const pageNumber = this._currentPageNumber;
-    // Non-numeric scale modes can be sensitive to the scroll orientation.
-    // Call this before re-scrolling to the current page, to ensure that any
-    // changes in scale don't move the current page.
-    if (isNaN(this._currentScaleValue)) {
-      this._setScale(this._currentScaleValue, this.isInPresentationMode);
-    }
-    this.scrollPageIntoView({ pageNumber, });
-    this.update();
   }
 
   _updateScrollModeClasses() {
@@ -1047,16 +1031,10 @@ class BaseViewer {
   }
 
   setSpreadMode(mode) {
-    if (mode === this.spreadMode) {
-      return;
-    }
     if (!Number.isInteger(mode) || !Object.values(SpreadMode).includes(mode)) {
       throw new Error(`Invalid spread mode: ${mode}`);
     }
-
     this.spreadMode = mode;
-    this.eventBus.dispatch('spreadmodechanged', { mode, });
-    this._regroupSpreads();
   }
 
   _regroupSpreads() {

web/pdf_viewer.js

@@ -87,6 +87,26 @@ class PDFViewer extends BaseViewer {
     });
   }
 
+  setScrollMode(mode) {
+    if (mode === this.scrollMode) {
+      return;
+    }
+    super.setScrollMode(mode);
+
+    this.eventBus.dispatch('scrollmodechanged', { mode, });
+    this._updateScrollModeClasses();
+
+    const pageNumber = this._currentPageNumber;
+    // Non-numeric scale modes can be sensitive to the scroll orientation.
+    // Call this before re-scrolling to the current page, to ensure that any
+    // changes in scale don't move the current page.
+    if (isNaN(this._currentScaleValue)) {
+      this._setScale(this._currentScaleValue, this.isInPresentationMode);
+    }
+    this.scrollPageIntoView({ pageNumber, });
+    this.update();
+  }
+
   _updateScrollModeClasses() {
     const { scrollMode, viewer, } = this;
 
@@ -102,6 +122,16 @@ class PDFViewer extends BaseViewer {
     }
   }
 
+  setSpreadMode(mode) {
+    if (mode === this.spreadMode) {
+      return;
+    }
+    super.setSpreadMode(mode);
+
+    this.eventBus.dispatch('spreadmodechanged', { mode, });
+    this._regroupSpreads();
+  }
+
   _regroupSpreads() {
     const viewer = this.viewer, pages = this._pages;
     // Temporarily remove all the pages from the DOM.