Description:
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When cropping an image suddenly a black line appears at the bottom of the image. To demonstrate this you can have a look at https://i.imgur.com/2G6J9iu.png where i combined the cropped and non-cropped in the same image. Of course you can not see the original image borders in this representation. So if it's not clear to you, just run the test script, twice maybe with $img = imagecropauto($img); commented out a second time. I tried viewing with IrfanView 4.37 and Paint.NET 4.0.6

Test script:
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<?php
$img = imagecreatetruecolor(400, 400);
imagefill($img, 0, 0, imagecolorallocate($img, 255, 255, 255)); // white background
$blue = imagecolorallocate($img, 0, 0, 255);
foreach (range(1, 5) as $font) {
  imagestring($img, $font, 10, $font * 15, 'test', $blue);
}
$img = imagecropauto($img);
imagepng($img, 'test.png');