The "New window" action/button is available in the standard toolbar mode (through the "Window" menu) and the "Tabbed" and "Tabbed compact" toolbar modes (in the "View" tab)... just not in the "GroupedBar compact" UI mode.
This feature is extremely useful to me to be able to edit multiple sheets at once in a spreadsheet, or multiple chapters within a monolithic text document, on multiple screens. Unfortunately, it cannot be used when the toolbar mode is "GroupedBar compact", which happens to be my favorite UI mode...
Andreas, up to you.
andreas kainz committed a patch related to this issue.
It has been pushed to "master":
tdf#132008 Provide NewWindow in Groupedbar compact view dropdown
It will be available in 7.0.0.
The patch should be included in the daily builds available at
https://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More
information about daily builds can be found at:
Affected users are encouraged to test the fix and report feedback.