[Regression] Ensure that pre-rendering of the next/previous page work…

…s correctly in Presentation Mode, when horizontal scrolling was enabled

Note how in `BaseViewer.forceRendering` the Scroll mode is used to determine how pre-rendering will work. Currently this is broken in Presentation Mode, if horizontal scrolling was enabled prior to entering fullscreen.

Furthermore, there's a few additional cases where the `this.scrollMode === ScrollMode.HORIZONTAL` check is pointless either in Presentation Mode or when a `PDFSinglePageViewer` instance is used.

web/base_viewer.js

@@ -597,11 +597,11 @@ class BaseViewer {
       if (!currentPage) {
         return;
       }
-      let hPadding = (this.isInPresentationMode || this.removePageBorders) ?
-        0 : SCROLLBAR_PADDING;
-      let vPadding = (this.isInPresentationMode || this.removePageBorders) ?
-        0 : VERTICAL_PADDING;
-      if (this.scrollMode === ScrollMode.HORIZONTAL) {
+      const noPadding = (this.isInPresentationMode || this.removePageBorders);
+      let hPadding = noPadding ? 0 : SCROLLBAR_PADDING;
+      let vPadding = noPadding ? 0 : VERTICAL_PADDING;
+
+      if (!noPadding && this._isScrollModeHorizontal) {
         const temp = hPadding;
         hPadding = vPadding;
         vPadding = temp;
@@ -834,6 +834,10 @@ class BaseViewer {
     this.container.focus();
   }
 
+  get _isScrollModeHorizontal() {
+    throw new Error('Not implemented: _isScrollModeHorizontal');
+  }
+
   get isInPresentationMode() {
     return this.presentationModeState === PresentationModeState.FULLSCREEN;
   }
@@ -906,8 +910,8 @@ class BaseViewer {
 
   forceRendering(currentlyVisiblePages) {
     let visiblePages = currentlyVisiblePages || this._getVisiblePages();
-    let scrollAhead = this.scrollMode === ScrollMode.HORIZONTAL ?
-      this.scroll.right : this.scroll.down;
+    let scrollAhead = (this._isScrollModeHorizontal ?
+                       this.scroll.right : this.scroll.down);
     let pageView = this.renderingQueue.getHighestPriority(visiblePages,
                                                           this._pages,
                                                           scrollAhead);

web/pdf_single_page_viewer.js

@@ -142,6 +142,11 @@ class PDFSinglePageViewer extends BaseViewer {
       location: this._location,
     });
   }
+
+  get _isScrollModeHorizontal() {
+    // The Scroll/Spread modes are never used in `PDFSinglePageViewer`.
+    return shadow(this, '_isScrollModeHorizontal', false);
+  }
 }
 
 export {

web/pdf_viewer.js

@@ -23,7 +23,7 @@ class PDFViewer extends BaseViewer {
   }
 
   _scrollIntoView({ pageDiv, pageSpot = null, }) {
-    if (!pageSpot) {
+    if (!pageSpot && !this.isInPresentationMode) {
       const left = pageDiv.offsetLeft + pageDiv.clientLeft;
       const right = left + pageDiv.clientWidth;
       const { scrollLeft, clientWidth, } = this.container;
@@ -87,6 +87,13 @@ class PDFViewer extends BaseViewer {
     });
   }
 
+  get _isScrollModeHorizontal() {
+    // Used to ensure that pre-rendering of the next/previous page works
+    // correctly, since Scroll/Spread modes are ignored in Presentation Mode.
+    return (this.isInPresentationMode ?
+            false : this.scrollMode === ScrollMode.HORIZONTAL);
+  }
+
   setScrollMode(mode) {
     if (mode === this.scrollMode) {
       return;