Reproduction steps:
1. Create a new Unity scene.
2. Create a new canvas and a UI element (example, a button)
3. Select the button, and using the rect tool, drag the resize handle on
the canvas to preview different canvas sizes.
4. Release the canvas, letting it snap back to its original position.
5. Select button and move mouse in scene.

Expected result: Nothing happens.
Actual result: Message "Look rotation viewing vector is zero" appears, huge performance loss (CPU usage increases) when button is selected, Canvas frame disappear.

Note: This problem affects every object that selected canvas has.
Profile shows that this problem is caused by "DockArea,OnGUI()"

Reproduced on: 5.5.0b7, 5.5.1p4, 5.6.0b9.
Works fine in: 5.4.4f1.
Regression introduced in 5.5.