Let the viewer close when Windows is shutting down - #778
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OnFormClosing cancelled every close, whatever the CloseReason. For a close the user asked for that is right: whether closing means hide or exit is ViewerProgram's rule and it needs a tray check to decide, so the form defers and CloseForReal brings the answer back. For the close Windows sends when the session is ending it is not. WinForms answers WM_QUERYENDSESSION with !e.Cancel, so a viewer that was open at shutdown told Windows it was preventing one, and the user got the "DiffEngineViewer is preventing shutdown" screen. With a tray running it was worse than a prompt: the loop's answer to a close request is to hide the window, so the process stayed up and went on blocking until the user chose "Shut down anyway". Do not cancel when the reason is WindowsShutDown or TaskManagerClosing. Both mean the process is going away regardless, so refusing buys nothing. Letting the close through is safe: the loop already watches for a disposed form and returns, running the same shutdown it would have run anyway.
This was referenced Aug 26, 2026
This was referenced Aug 27, 2026
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OnFormClosing cancelled every close, whatever the CloseReason. For a close the user asked for that is right: whether closing means hide or exit is ViewerProgram's rule and it needs a tray check to decide, so the form defers and CloseForReal brings the answer back.
For the close Windows sends when the session is ending it is not. WinForms answers WM_QUERYENDSESSION with !e.Cancel, so a viewer that was open at shutdown told Windows it was preventing one, and the user got the "DiffEngineViewer is preventing shutdown" screen. With a tray running it was worse than a prompt: the loop's answer to a close request is to hide the window, so the process stayed up and went on blocking until the user chose "Shut down anyway".
Do not cancel when the reason is WindowsShutDown or TaskManagerClosing. Both mean the process is going away regardless, so refusing buys nothing. Letting the close through is safe: the loop already watches for a disposed form and returns, running the same shutdown it would have run anyway.