This issue is specific to Safari on iOS. It doesn't happen in Safari on macOS, nor in other browsers.
We (noVNC) have a canvas where we grab all UI events (keyboard, mouse, touch), pass them on to the remote system, and call preventDefault() to avoid both a local and remote action.
For the most part, this works well, but not for selections on iOS. There are two primary cases we've seen:
* Pressing Cmd+A
In these cases, we get a local selection in Safari as well, which means we get a blue overlay annoying the user.
We're trying to work around most of it by using -webkit-user-select, but it is difficult to handle all cases.