The title of the bug report is a bit misleading, but well... so is Qt Creator's behavior.

Namely:
1) Make a new project in Creator
2) create a new file e.g. 1.cpp file outside Creator
3) now right-click on the project name in the side pane and try to "Add existing file" to your project: surprise surprise! No can do: "Add existing file, Add new file, Add library" are all disabled.

Now, if you are lucky enough to be persistent, just right-click on the project name ONE MORE TIME, and there you are: the previously-disabled items guessed your frustration and magically became enabled.

Screenshot: wrongly_disabled_items.jpg

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PS
I only discovered this workaround (i.e. "just right-click once more, you dummy!") by accident after over two weeks of playing with Creator, and my guess is many newcomers to Creator who'll have this problem will just dump Creator, curse a lil' to blow their steam, and go right back to their old ways