Starting with v.3.6.0 beta 1, a previously working GPIW, is suddenly closing unexpectedly after a player performs any keystrokes or right-click operations on pieces in that window. I suspect the unexpected behavior was introduced with 10001: Inventory window allows a hotkey for Refresh. I verified it still occurs on 3.6.4. It was working in all previous versions of 3.4 and 3.5.
I think I found a workaround for my issue. It seems that GPIWs did not open/close or refresh properly in previous Vassal versions so I used a shotgun approach to send triggers to close GPIWs. That behavior seems to be fixed now so I simply removed all the excess commands and the window now closes as expected. The behavior is definitely different before/after 3.6.0 beta 1.