OK, I see what the problem is: the game record provides the FEN tag before the Variant tag.
So when the game is loaded while WinBoard is not yet in FRC mode, the Variant tag will
trigger a variant switch to FRC, which will also trigger a new shuffling of the start position.
This has only a 1-in-96 chance of reproducing the position from the FEN tag, and in all
other cases the following moves will no longer fit the start position.
If I change the order of these tags, the game loads without problems.
If WinBoard is set to FRC first, through the New Variant dialog, the game also loads
without problems, irrespective of the tag order. This should be considered a work-around
for the mentioned problem.
I guess loading a game of a variant that is not currently selected is bound to cause trouble
if the Variant tag does not come before other variant-specific tags, such as the FEN tag.
Because the FEN would be parsed in the wrong context: piece IDs in the FEN can have different
meaning in different variants, and even the board size could be different, or extra fields
could be present (e.g. for recording the number of remaining checks in 3-checks) which
would get ignored.
Now the Variant tag is an extension to the PGN standard, and there is thus no official standard
that explicitly declares the ordering of the tags as used by TCEC as illegal. Although a lot
of misery could be avoided by declaring it is. Nevertheless, for the convenience of being
able to automatically load such 'illogically encoded' games (e.g. when using the file
association for loading the game by double-clicking the PGN file icon), it would be better
to defer processing of a FEN tag until move number 1 is encountered.
Op di., mrt. 1, 2022 om 20:47, Tim Mann <tim@tim-mann.org> schreef:
I'm not really working on xboard or WinBoard anymore. I'm cc'ing the developers mailing list on this reply in case anyone else can help.
Hello, Tim Mann,
I really like your WinBoard program. I use Winboard 4.2.7 and Winboard 4.8.0 to watch games by downloading and opening the archive in pgn format. On the website
https://tcec-chess.com/#x=archive there is an archive of computer engine games, for example, Season 21, where FRC 4 is Fischer Chess 960: Winboard 4.2.7 Winboard 4.8.0 does not show Fischer chess 960 games, they write an error message "Illegal move" on moves 0-0; 0-0-0 and other moves if there are comments in parentheses in the pgn text. In previous seasons, computer engines also played Fischer's 960 chess, and Winboard also writes an error message "Illegal move" on moves 0-0; 0-0-0. Please fix these Winboard errors to be able to view Fischer's 960 chess games.
PS: Archive of people's games in png format from the site
chess-poster.com WinBoard shows without errors.
Best wishes,
Vadim Trofimov
from Russia