Whether I press F11 or select View - Styles, the Styles window appears for half a second and disappears. I first thought it was because of my text file, which was rather complex, but then I also tried with a simple file on which I had used this function in the past, and it did the same. I also tried with a new empty text file. Same problem. I also used the Repair function on the Windows Uninstall panel, but this didn’t help.
I have noticed this problem recently. I found that VIEW > TOOLBAR LAYOUT > SIDEBAR brought the sidebar back.
Yes, thanks for this tip. Selecting any toolbaar layout solves the problem, after which the Styles window can open with View - Styles. F11 spoils it again. So don’t use F11 until this bug is solved.
Having done as @petermau suggested, and having docked the toolbar F11 worked for me also under View > Toolbar Layout > Default.
Newly undocking it made the bug malign again.