B767 FLT CREW EXPRESSED CONCERN REGARDING SEPARATION WHEN XING RWY AT DFW AND DEP OVERFLEW HIS ACFT.

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WE LANDED AT DFW ON RWY 36R AND HAD TO TAXI TO THE COMPLETE OPPOSITE SIDE OF THE ARPT FOR PARKING. AS WE TAXIED ACROSS THE BRIDGE; WE WERE TOLD BY THE GND CTLR TO HOLD SHORT OF RWY 36L. WE DID. AFTER A WHILE; THE CTLR TOLD US THAT DIRECTLY ACROSS FROM US WAS A B757. WHEN HE CLRED THE RWY AND TRANSITIONED TO THE NEXT TXWY N OF US; WE WERE CLRED TO CROSS THE RWY AND TAXI TO OUR RAMP. WE WATCHED AS HE CROSSED THE RWY AT A VERY SLOW SPD; AND THEN AS HE TRANSITIONED TO THE PROPER TXWY. WHEN CLR; I TURNED ON OUR STROBE AND LNDG LIGHTS AS I DO WHEN XING A RWY. THE FO SAID 'CLR R' AS HE SCANNED THE RWY ON HIS SIDE; AND I SAID 'ONE IN POS' AS I NOTED THE PLANE AT THE DEP END; ABOUT 2 MI FROM OUR POS. AS WE WERE CLRING; I NOTICED THAT THIS PLANE WAS NOW FLYING OVER US AND HAD TAKEN OFF WHILE WE CROSSED THE RWY. WE REVIEWED OUR COMS AND OUR READBACKS OF ALL HOLDING INSTRUCTIONS AND DECIDED THAT MAYBE THE CTLRS HAD MADE A MISTAKE IN CLRING HIM FOR TKOF WITHOUT ENSURING THAT WE WERE CLR OF THE RWY. THE SLOW TAXI OF THE B757 MAY HAVE MADE THEM ASSUME THAT WE WERE CLR WHEN WE WERE NOT. THE GND CTLRS NEVER MADE MENTION OF THIS SIT AT ALL. WE DID NOT RAISE ANY QUESTIONS BUT JUST TAXIED IN.