I have this same problem. Just upgraded from 2025 last night: no notation or lead sheet. The other windows (tracks, chords, etc. all work fine).
I think the issue is to do with multiple displays. @dimtass do you have multiple displays?

I tried Ctrl+Click and discovered that these DO open in new windows - on my laptop monitor, when the main program window is on my extended external monitor.
So, I unplugged the monitor and now my notation windows display in the main UI.
Also, If I change the external monitor from "extended" to "main" in the Mac settings, then I see what I expect - notation/lead sheet in the main UI.
With this working, I switched back to the way I want, with the laptop screen as main and external monitor as extended. The notation window persisted, BUT when I closed and reopened it (Shift+Click) it did not display again.

Another data point for your troubleshooting: When I go to mission control, it shows that the notation window is somehow active, but is not visible otherwise (see large screenshot).
And, I get a warning when opening the lead sheet/notation - not intentionally in a separate/floating window, just clicking on it, that the display has been moved to a different monitor (see small screenshot).
Thanks in advance for investigating this.

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BIAB 2026 for Mac (started with BIAB 2006 for Windows)
Guitarist and Worship Leader from Townsville, Australia