Two problems in the same area reported:

1. Single dirty file is directly saved during the OS restart without asking the user 1st (Community report1)
2. N++, with its backup engine ON, does not behave identically during the usual N++ exit initiated by the user and during the one initiated by the OS restart (Community report2}

STRs:

• open only 1 existing file in N++
• add some text in the opened file without saving it, do not close N++
• restart Windows, “This program preventing restart...” dialog appears, click on the "Cancel" option there - the opened unsaved file is saved now (error - the N++ SaveAll dialog should be opened instead as in the case of 2 or more dirty files)
• this problem has to be fixed for sure, N++ should behave the same as for the 2 or more dirty files...

• “Remember current session for next launch” and “Enable session snapshot and periodic backup” are enabled in the N++ 'Preferences'
• open some files, edit at least one of them
• now if the running N++ was closed by user manually ... no save dialog for the changed file(s), N++ just silently closes and on its next start all the files are opened again from the N++ backup
• but if the Windows OS restart tried to close that running N++ ... we get the “This program preventing restart...” dialog and when the user clicks on its "Cancel" option, N++ offers its SaveAll dialog for all the unsaved files ("Restart anyway" option is ok from this point of view)
• this is intentional new safer behavior at the OS restart time, but it can be changed back to the previous state, when only the N++ backup engine silently handles the unsaved user's data

Possible solutions:

• either we create a new separate N++ message (e.g. IDM_FILE_SAVEALL_AT_SHUTDOWN) and use it here, then show in its handler new YESNO dialog “Windows restart/shutdown is closing the Notepad++ running. Do you want to save unsaved file(s)?” etc.
• or we adjust the v8.4.3+ speed exception for the only one dirty file in the Notepad_plus::fileSaveAll() at the OS restart time to: if ((nbDirty == 1) && curBuf->isDirty() && !(NppParameters::getInstance()).isEndSessionStarted())

If the consistency-view wins, we can use an additional check for the N++ active backup here like:

// we have unsaved filebuffer(s), give the user a chance to respond (non-critical shutdown and N++ backup-OFF only)
if (!isForcedShuttingDown && isFirstQueryEndSession && !nppParam.getNppGUI().isSnapshotMode())