download button disappear outside facebook photo

Browser: Google Chrome 51.0.2704.106

Helper version: 6.42

Some photos in facebook are not wide enough, so facebook adds some black padding on both left and right of the photo. When we hover the photo, the download button appears on the top left corner where the black padding is. However, when the mouse goes into that area, the download button disappears. Therefore, it's not possible to click the button for such photos.