fix a host of isues with reset: #4121, #4125 and #4277

src/InteractiveWindow/Editor/InteractiveWindow_UIThread.cs

@@ -334,6 +334,12 @@ private void PendSubmissions(IEnumerable<PendingSubmission> inputs)
 
             public void AddInput(string command)
             {
+                // If the language buffer is readonly then input can not be added. Return immediately.
+                if (_window._currentLanguageBuffer != null && _window._currentLanguageBuffer.IsReadOnly(0))
+                {
+                    return;
+                }
+
                 if (State == State.ExecutingInput || _window._currentLanguageBuffer == null)
                 {
                     AddLanguageBuffer();