Bug 1657550 - Preserve x-axis scroll position on printPreviewScrollToPage call in any cases. r=emilio

That's what we've done for PRINTPREVIEW_PREV_PAGE, PRINTPREVIEW_NEXT_PAGE
and PRINTPREVIEW_GOTO_PAGENUM.

Differential Revision: https://phabricator.services.mozilla.com/D87545

layout/base/nsDocumentViewer.cpp

@@ -3329,17 +3329,21 @@ nsDocumentViewer::PrintPreviewScrollToPage(int16_t aType, int32_t aPageNum) {
       mPrintJob->GetPrintPreviewPresShell()->GetRootScrollFrameAsScrollable();
   if (!sf) return NS_OK;
 
+  // Figure where we are currently scrolled to
+  nsPoint currentScrollPosition = sf->GetScrollPosition();
+
   // Check to see if we can short circut scrolling to the top
   if (aType == nsIWebBrowserPrint::PRINTPREVIEW_HOME ||
       (aType == nsIWebBrowserPrint::PRINTPREVIEW_GOTO_PAGENUM &&
        aPageNum == 1)) {
-    sf->ScrollTo(nsPoint(0, 0), ScrollMode::Instant);
+    sf->ScrollTo(nsPoint(currentScrollPosition.x, 0), ScrollMode::Instant);
     return NS_OK;
   }
 
   // If it is "End" then just scroll to the `scrollTopMax` position.
   if (aType == nsIWebBrowserPrint::PRINTPREVIEW_END) {
-    sf->ScrollTo(nsPoint(0, sf->GetScrollRange().YMost()), ScrollMode::Instant);
+    sf->ScrollTo(nsPoint(currentScrollPosition.x, sf->GetScrollRange().YMost()),
+                 ScrollMode::Instant);
     return NS_OK;
   }
 
@@ -3349,9 +3353,6 @@ nsDocumentViewer::PrintPreviewScrollToPage(int16_t aType, int32_t aPageNum) {
     return NS_ERROR_FAILURE;
   }
 
-  // Figure where we are currently scrolled to
-  nsPoint currentScrollPosition = sf->GetScrollPosition();
-
   int32_t pageNum = 1;
   nsIFrame* fndPageFrame = nullptr;
   nsIFrame* currentPage = nullptr;