Leaky Shower Door
The threshold looks pretty flat, there should be a taper back to the shower pan. Also the threshold tile is pieced and there appears to be a grout line right at the door frame this looks to be acting like a little dam. it is really best to have a solid piece on the top of the threshold and tapered back to the pan then you should be in good shape. So try to find a solid white marble or other solid surface threshold, tear off the tile and install the marble with proper pitch 1/4 per foot minimum. Or buy yourself a new door that has an integral waterproof threshold that could be installed over the existing tile.