Double-clicking on a Category symbol in the Symbology table does not open the Symbol Selector window if ArcMap is open on the First Monitor and the Second Monitor is set as the 'main display'.
1. Open ArcMap on the 'main display' monitor.2. Make the first monitor the 'main display' monitor.3. In the Symbology Tab select the symbol and click 'Symbol' > 'Properties for selected symbols'.4. Double click the symbol in the table of contents.