To reproduce:
1. Open attached Unity project "ClickTest2.zip"
2. Open "Game_0" scene
3. Go into Play Mode
4. In the Hierarchy, navigate to Canvas --> Panel --> StartPanel
5. Set x and y scale of the StartPanel object to 0

Expected result: StartPanel is scaled to 0, so the raycast should not be blocked by it and the buttons should be possible to click
Actual result: The raycast is blocked, buttons are impossible to click.

Note: Scaling the z axis to 0 fixes the issue and the raycast is no longer getting blocked

Regression first introduced in: 2017.1.0b1
Reproduced on: 2017.1.0b1, 2017.1.1f1, 2017.2.0b11, 2017.3.0a7
Not reproduced on: 5.6.3p2

Note: in versions 2017.1.0b1 - 2017.1.0b8 setting z axis scale to 0 and trying to click on a button prints an "Assertion failed: Assertion failed on expression: 'IsNormalized(dir)' " error to console