When I flush my toilet, the water level will drop about an inch after the tank fills and then fill up again. This goes on non-stop UNTIL I turn the supply off. If I turn the supply off immediately after the tank fills, then the water level does NOT drop. I have replaced the flapper, adjusted the flapper, and disassembled the fill valve in the tank to check for calcium deposits. I have cleaned the seal under the flapper. I have put petroleum jelly on the flapper. It does not make sense that the leak does not happen if I turn the supply off immediately after the tank fills. Any ideas as to what is going on and how to fix it?
This sounds like a case with the bowl fill line that refills the bowl while the tank is refilling is the problem.
What may be happening is this line is low enough in the tube and when the float stops the incoming water flow it starts a siphon from the tank and once low enough it will refill the tank and start the cycle again.
The fix is cutting the tube off at the top of the pipe above the water line this will usually stop the siphon.
Many new refill valves actually have a clip that holds the tubing about a ~1/2” above the tube to stop this from happening.
Try ether cutting the tubing shorter or raising it above the water line and that should stop the siphon.