While using Gmail, you may receive an email that has a large embedded (such as 1200 pixels wide). The top of your screen will resize as expected and all account controls will show up.
However, the image will not be resized. As a result, the message window (where the message text shows) will be the full fixed width of the image. This will push the reply button and other controls to the far right beyond your viewable window (unless you’re on a high resolution display).
This problem would not occur if the message was drafted within Gmail, but when email clients are used (such as Outlook or Apple Mail) they may use HTML formatting and set a fixed width for the embedded image.