I recently added a popup to my app/website warning users of a coming maintenance window. On phones, it appeared as a little window, slightly smaller than the webpage beneath it. The popup had an "OK" button at the bottom to close the window.
The complete message and button would normally show, but, in some cases, people had set their device font so large that the button was pushed down and didn't appear. The popup allowed scrolling, but some people weren't aware that you could scroll in it. They read the message and then didn't know what to do. They were stuck.
How do I avoid this? Should a popup always be closeable from a top X? I would like for people to read the message. Or is there a good way to indicate scrolling on a mobile device? Or maybe popups on phones should always be full-page instead of little windows on top of a page since people know to scroll on pages?