diff --git a/core/modules/system/src/Plugin/ImageToolkit/Operation/gd/Rotate.php b/core/modules/system/src/Plugin/ImageToolkit/Operation/gd/Rotate.php
index 0860b11..98561d9 100644
--- a/core/modules/system/src/Plugin/ImageToolkit/Operation/gd/Rotate.php
+++ b/core/modules/system/src/Plugin/ImageToolkit/Operation/gd/Rotate.php
@@ -40,14 +40,10 @@ protected function arguments() {
    * {@inheritdoc}
    */
   protected function validateArguments(array $arguments) {
-    // Convert any negative angle to a positive angle. Reason:
-    // PHP 5.5 rotates in an unexpected manner. The resulting image will be 1
-    // pixel smaller in both dimensions then it would be in PHP 5.4 and what
-    // math teaches us. Only positive multiples of 90 degrees are rotated as
-    // one would expect.
-    if ($arguments['degrees'] < 0) {
-      $arguments['degrees'] -= floor($arguments['degrees'] / 360) * 360;
-    }
+    // PHP 5.5 GD bug: https://bugs.php.net/bug.php?id=65148: To prevent buggy
+    // behavior on negative multiples of 90 degrees we convert any negative
+    // angle to a positive one between 0 and 360 degrees.
+    $arguments['degrees'] -= floor($arguments['degrees'] / 360) * 360;
     return $arguments;
   }
 
diff --git a/core/modules/system/src/Tests/Image/ToolkitGdTest.php b/core/modules/system/src/Tests/Image/ToolkitGdTest.php
index e2c3a31..c966814 100644
--- a/core/modules/system/src/Tests/Image/ToolkitGdTest.php
+++ b/core/modules/system/src/Tests/Image/ToolkitGdTest.php
@@ -257,11 +257,16 @@ function testManipulations() {
         $correct_dimensions_real = TRUE;
         $correct_dimensions_object = TRUE;
 
-        // PHP 5.5 rotates in an unexpected manner. The resulting image will be
-        // 1 pixel smaller in both dimensions then it would be in PHP 5.4 and
-        // what math teaches us. Only (positive) multiples of 90 degrees are
-        // rotated as one would expect (but the GD rotate operation already
-        // converts negative angles to positive ones).
+        // PHP 5.5 GD bug: https://bugs.php.net/bug.php?id=65148. PHP 5.5 GD
+        // rotates differently then it did in PHP 5.4 resulting in different
+        // dimensions then what math teaches us. For the test images, the
+        // dimensions will be 1 pixel smaller in both dimensions (though other
+        // tests have shown a difference of 0 to 3 pixels in both dimensions.
+        // @todo: if and when the PHP bug gets solved, add an upper limit
+        //   version check.
+        // @todo: in [#1551686] the dimension calculations for rotation are
+        //   reworked. That issue should also check if these tests can be made
+        //   more robust.
         if (version_compare(PHP_VERSION, '5.5', '>=') && $values['function'] === 'rotate' && $values['arguments']['degrees'] % 90 != 0) {
           $values['height']--;
           $values['width']--;
@@ -294,16 +299,16 @@ function testManipulations() {
                 $y = 0;
                 break;
               case 1:
-                $x = $values['width'] - 1;
+                $x = $image->getWidth() - 1;
                 $y = 0;
                 break;
               case 2:
-                $x = $values['width'] - 1;
-                $y = $values['height'] - 1;
+                $x = $image->getWidth() - 1;
+                $y = $image->getHeight() - 1;
                 break;
               case 3:
                 $x = 0;
-                $y = $values['height'] - 1;
+                $y = $image->getHeight() - 1;
                 break;
             }
             $color = $this->getPixelColor($image, $x, $y);