As much as I can remember the window for Opera10 has always been a bit "oversized" when using FLWM. That does not appear to be new for TC 2.11rcX and IIRC has been reported before.
I guess in the initial "negotiation" between Opera10 and FLWM something goes wrong, insofar that the window size and position in my case gets saved with the following values: tc@box:~$ grep 'Browser Window' ~/.opera/operaprefs.ini
Browser Window=-3,-18,1024,768,0
The consequence appears to be that the window bar is outside the visible screen (-3,-18 ??)
When using 'opera.tcz' (i.e. v9.64) the respective information is different: tc@box:~$ grep 'Browser Window' ~/.opera/opera6.ini
Browser Window=-1,-1,1017,746,0
If one would have used a different WM (e.g. 'jwm') during this first time Opera10 was run, the information would have been different again (e.g. Browser Window=0,0,1016,748,2). In in all the other cases the window bar would have been "inside the screen".
So I guess if one does not like this one could either change the windows size with the "special keys" for FLWM (e.g. Ctrl-Alt-s and Ctrl-Alt-,) or you could alter the respective line in '~/.opera/operaprefs.ini'.