When there is a big value (starts around 150px) set for Max avatar dimension it causes an overflow in the cropper tool used for editing avatars. The issue seems to be caused by these two lines of code found in phpbb_avatars.js

                minContainerHeight: this.allowedSizes.height.max * 2, // Double max size for better usability

                minContainerWidth: this.allowedSizes.width.max * 2, // Double max size for better usability