Bug 85686 - Mimic Chromium's getSelection().toString() behaviour r=masayuki,dom-core,webidl,smaug

Basically when getSelection().toString() is called, Chromium may
return an serialization on a different content than Firefox.
This patch tries to address this webcompat issue by mimicing
the same behaviour.

We should still address the spec issues, but this seems to be
an acceptable compromise.

Differential Revision: https://phabricator.services.mozilla.com/D239657

layout/generic/nsFrameSelection.cpp

@@ -1408,7 +1408,10 @@ nsresult nsFrameSelection::TakeFocus(nsIContent& aNewFocus,
            __FUNCTION__, &aNewFocus, aContentOffset, aContentEndOffset,
            static_cast<int>(aHint), static_cast<int>(aFocusMode)));
 
-  mPresShell->FrameSelectionWillTakeFocus(*this);
+  mPresShell->FrameSelectionWillTakeFocus(
+      *this, aNewFocus.CanStartSelectionAsWebCompatHack()
+                 ? PresShell::CanMoveLastSelectionForToString::Yes
+                 : PresShell::CanMoveLastSelectionForToString::No);
 
   // Clear all table selection data
   mTableSelection.mMode = TableSelectionMode::None;
@@ -3115,7 +3118,16 @@ void nsFrameSelection::DisconnectFromPresShell() {
     MOZ_ASSERT(mDomSelections[i]);
     mDomSelections[i]->Clear(nullptr);
   }
-  mPresShell = nullptr;
+
+  if (auto* presshell = mPresShell) {
+    if (const nsFrameSelection* sel =
+            presshell->GetLastSelectionForToString()) {
+      if (sel == this) {
+        presshell->UpdateLastSelectionForToString(nullptr);
+      }
+    }
+    mPresShell = nullptr;
+  }
 }
 
 #ifdef XP_MACOSX