Reproduction steps:
1. Open project attached.
2. Open scene "Test".
3. Press on an object that has written "Prefab 2 No Button" on it.
4. Press on an object that has written "Prefab 1 Button" on it.

Expected result: In both cases in the hierarchy the root of prefab will be selected.
Actual result: When pressing on prefab with button, it selects button instead of prefab.

Reproduced on: 5.5.0f3, 2017.1.0p5, 2017.2.0b9, 2017.3.0a6.

There is a link to documentation that states "If you click on an object that is part of a prefab, the root of the prefab is selected because a prefab root is treated as a selection base."
https://docs.unity3d.com/ScriptReference/SelectionBaseAttribute.html