tdf#94679 Writer: fix lost selection with Shift-PageDown

Push/Pop-ing the cursor led to selection loss.
SelectHiddenRange() immediately returns false if the current cursor
hasMark(), so avoid all of the bugs and expensive push/pop routines
when there is a mark already.

Change-Id: I4624a3e0b2267942812d0429d527ad97962ec7fc
Reviewed-on: https://gerrit.libreoffice.org/19108Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

sw/source/uibase/shells/textsh.cxx

@@ -660,9 +660,13 @@ void SwTextShell::StateInsert( SfxItemSet &rSet )
     SfxObjectCreateMode eCreateMode =
                         GetView().GetDocShell()->GetCreateMode();
 
-    rSh.Push();
-    const bool bCrsrInHidden = rSh.SelectHiddenRange();
-    rSh.Pop();
+    bool bCrsrInHidden = false;
+    if( !rSh.HasMark() )
+    {
+        rSh.Push();
+        bCrsrInHidden = rSh.SelectHiddenRange();
+        rSh.Pop();
+    }
 
     while ( nWhich )
     {