In a new UserControl, in XAML Intellisense was not working but on MainWindow it was working just fine as with the C# codebehind.
However, if I press CTRL + {space} it still comes up fine.
I went to Tools➾Options➾Text Editor➾XAML➾General, and the Text Completion section is greyed out (e.g. Auto-List Members, etc.). This is also greyed out when I see it on my main window, but Intellisense works there.
How can this be resolved?
Huh. I just restarted it and it worked again. The problem is that before, when I restarted it, it still did not work. I haven't found the underlying issue, but it's fixed now.