Bug 1712355 - Modal dialogs are broken with Fission+BFCache, r=peterv

This fixes webdriver/tests/forward/user_prompts.py

Differential Revision: https://phabricator.services.mozilla.com/D115761

docshell/base/nsDocShell.cpp

@@ -1190,6 +1190,7 @@ void nsDocShell::FirePageHideShowNonRecursive(bool aShow) {
   // For pagehide, set mFiredUnloadEvent to true, so that unload doesn't fire.
   nsCOMPtr<nsIContentViewer> contentViewer(mContentViewer);
   if (aShow) {
+    contentViewer->SetIsHidden(false);
     mRefreshURIList = std::move(mBFCachedRefreshURIList);
     RefreshURIFromQueue();
     mFiredUnloadEvent = false;