UI Button positioned outside Panel parent not registering click event (Unity 5.3.4)

I have a weird problem with a UI Button:

The UI button is the child of a UI panel. The panel is positioned outside of the screen. The button itself is the only thing visible on screen and therefore positioned outside of its parent panel (and anchored to one corner of the panel).
When the button is clicked, the whole panel plus button moves up (on screen) and with another click it moves down (outside of screen, only button visible).

Last time I worked on the project everything worked fine, but today the button does not register a click event anymore. I haven’t changed anything since then.

When I move the button object outside the panel in the hierarchy, so that they are not related anymore, it does work again. But I don’t get why…because two days ago it did also work when the button was a child to the panel. It has to be a child, so the button will move with the panel and is always relatively positioned to it.

I checked:
The code and animations work properly, I have a canvas, an event system and all other buttons work. Also the mentioned button is not covered by anything else and should be the last thing rendered.

Has anyone an idea what the cause of the probleme could be?!

Ok, i finally found the problem after reproducing the whole thing in a new project:

I had (for whatever unknown reason) a canvas component on my UI panel that did not serve any purpose. After removing the canvas component everything worked again, even if the button was a child of the panel.