Add test methods for non-covered parts:

• Constructor using startEmpty=true
• containsKey
• containsValue
• size
• isEmpty

maybe add a message like 'found additional initally set property, check factory method, see org.eclipse.scout.rt.client.ui.BasicPropertySupportFactory'

underlaying map is not per-se empty if it was initialized with default values?

why does the methods mix up two init pattern (new ImmutablePair() vs. defaultValues.put) ?
-> use defaultValues.put() for all?

use _ instead of ignored?

code is hard to read -> split into several methods or use if then else on separate lines and prepending a javadoc indicating which case is handled

e.g.:

  @Override
  public Object put(String key, Object value) {
    boolean additionalValuesContainsKey = m_additionalValues.containsKey(key);
    if (m_defaultValues.containsKey(key)) {
      if (isDefaultValue(key, value)) { // must be same, equals would not be sufficient
        if (additionalValuesContainsKey) {
          Object removedValue = m_additionalValues.remove(key);
          return removedValue != REMOVED_MARKER ? removedValue : null;
        } else {
          return m_defaultValues.get(key);
        }
      }
      else if (!additionalValuesContainsKey) {
        m_additionalValues.put(key, value);
        return m_defaultValues.get(key); // load previous value from default values (if any)
      }
    }
//....

extract into protected sub-class for maintenance/override?

check for missing coverage in DefaultValueMap:

• Iterator/EntrySet.next()
• Iterator/EntrySet.remove()
• Iterator/EntrySet.size
• Iterator/EntrySet.isEmpty

do we need a platformtestrunner if no BEANS are used?

suppress warnings