I'll take the devil's advocacy on this one... And preface by saying as a player I absolutely hate being in A's shoes on these angles, and have no visceral desire to protect players who do what B did here.
First: this is all very non-standard behavior when facing a bet so this is a Rule 1 decision.
Second: one version of the facts is that when facing a bet Player B 1) put his cards "in the middle of the table" AND 2) mumbled something that's not clear
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Now, since I don't know what this guy said, I'm really left with his ACTIONS. In GG's original post, what perplexes me the most is the line "in the middle of the table".... so, let's take two extremes:
Extreme 1: Player B retains control of his cards very near to his staging area of the table, he turns them up, but keeps his fingers on them... and he appears to be contemplating a bet....
OR Extreme 2: Player B casually tosses his cards face up right into the middle of the table, more or less exactly as he would if he were mucking... and mumbles something.
Since this is a Rule 1 decision, the closer the actions were to Extreme 1, the more likely I'm going to rule in B's favor... But the closer his actions are to Extreme 2, the more likely I'm going to rule in A's favor.
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If a guy clearly says "I call" or in this case even clearly says "I'll pay you" (a very non-standard declaration btw), THEN tosses his cards face up in the middle of the table, fair enough he's calling.
Even if he turns his cards up and pushes them forward towards the middle but retains finger contact with the cards (i.e. remains in control of them), I don't like that, but I'll rule his cards are still live...
But if he just tosses his cards face up in the middle of the table I'm very tempted to say "when facing a bet you tossed your cards forward, relinquishing control" and in the absence of any preceding clear statement, that's a fold.
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So there you have it GG.... 3 people's opinions.... how did you rule ?