Using Canvas on World Space or Camera, buttons are not clickable if its Pos Z is less than zero. Setting Pos Z to 1 fixes the issue.

To reproduce:
1.Open users attached project "TestCanvasInWorldSpace.zip"
2.Open "TestScene.unity" scene
3.In hierarchy on Canvas, make sure that Pos Z is set to less than 0, for e.g. -100
4.Enter play mode
5.Observe as you can not press the button
6.Exit play mode and set Canvas Pos Z to 1
7.Enter play mode
8.Observe as the button is clickable

Expected result: button is clickable even tho Pos Z on canvas is set to less than zero
Actual result: button is not clickable is canvas Pos Z is set to less than zero

Reproduced on: 2017.1.0f2 2017.1.0f3 2017.1.0p1 2017.1.0p2 2017.3.0a1
Not reproduced on: 5.6.2p4 2017.1.0a2 2017.1.0b1 2017.1.0b5 2017.1.0b8
Regression introduced in 2017.1.0b9