by a canvas

By an extremely short or slim margin (of distance, time, or another measure). The phrase comes from boat racing, as the front end of a racing boat was at one time covered with canvas. They're just about to close the gates! It looks like we made the flight by a canvas. The race was neck and neck till the very end, but Sally won it by a canvas. Ah, she lost the gold medal by a canvas when her leg cramped in the last few meters.

by a canvas

by a small margin.

The tapered front end of a racing boat was formerly covered with canvas to prevent water being taken on board. In this context, to win by a canvas meant to win by the length between the tip of the bow and the first oarsman.