In the Roundcube webmail web app, if you are using Safari on an iPad, the screen width is whatever it is, some random number in some parts of the system, and at times this random number determines whether or not you will be able to see some user interface elements or be able to click on certain objects.
I went into the server-side filters section where you can delete incoming junk for example, or cc certain classes of email to someone else for another example. I have done both. On one occasion I was trying to make changes to the latter. I was using an iPad 5 and guess what, the screen simply was not wide enough for me to see certain parts of the display on the right hand side, so in my particular case, adding more rules to cc certain kinds of email to other people is impossible.
How did I ever get it set up in the first case, I hear you ask? I can only think that I did it on my iPad Pro 12" which has a massive screen, but sometimes certain sites in Safari display stuff rescaled using giant pixels to make the number of x pixels the same as on a standard iPad such as an iPad6. That is evil. But it has to be that that did not happen in this case and for this idea to work the page would have to have been much larger in the sense of having the correct, much greater number of pixels available. So then the genius page authors have succeeded in making something that will become unusable in a different machine, and only by sheer fluke happens to work on one rare variant.
The whole thing needs to be redesigned from scratch in terms of layout: what is the point of drawing stuff so enormous so that it does not fit on the screen anyway. And if no other solution can be found, then the designers if the page and the Safari team need to make sure the user can scroll the page anyway! Why on earth is Safari preventing the user from simply scrolling the page so that the user can get to all of the content ?