I confirm the bug with the current SVN (r13400) as I described. The bug does not arise if the first window is showing an opened document and then steps 2),3) are executed. The second window will show the document but with the correct style.

I cannot reproduce Jeroen sequence since in my menus there is no "View->New window". However I can proceed a "No style" document as follows:
1) Start TeXmacs (opens "No name [XXX]")
2) File->New (opens a second window with "No name [XXX+1]")
3) In this second window select "Go->Close" (Buffers in this window)

At this point the second window show the buffer "No name [XXX]" as the first window but with "No style". Note that I can still edit the same buffer in the first window but there the style is correctly set to "Generic", for example a \section tag inserted in the buffer is correctly rendered in the first window but not in the second.

Fixed in revision 13389.

The following sequence of steps produces the bug with a fresh install of TeXmacs v1.99.18 (directory ~/.TeXmacs has been removed)

• close the welcome window 'Help—Welcome…' but keep the Untitled [1] window open
• file > new, creates an Untitled [2] window
• close Untitled [1]
• file > new, creates another Untitled [1] having the buggy "No-style" style.

Now I realize that the sequence of steps has some similarities with bug #59519: the two bugs might be related.

When a new document is created, this sometimes results in "Sans style" being displayed on the left of the contextual bar, instead of the expected "Générique" style (see attached picture). I could not find the pattern that governs this, it appears to be random. I will be happy to do more systematic testing if you cannot reproduce this.

The problem with "Sans style" is that many buttons do not work, e.g., the Title button displays <doc-data|<doc-title|…>> instead of the formatted title, because basic environments are undefined.