Steps to reproduce:

1. Open attached project
2. Open TestScene
3. Find "Content" gameObject under Canvas->DropDown->Template->Viewport
4. Grab the side of your Game Window to rescale it and give it a good thrashing (Scale up and down so the canvas re-sizes)
5. At this point the Content element should have gone missing,some or all of the position characteristics being "NaN" value

Expected outcome: The canvas dropdown element resizes along with the parenting dropdown container.
Actual outcome: Upon some up and down re-sizing, the element loses its relative position and goes off screen

Reproduced on 5.6.0b3, 5.6.0b5, 5.6.0b8, 5.6.0b11, 5.6.0f1, 5.6.0f2, 5.6.0f3
Regression introduced in 5.6.0b2
Works fine on 5.3.3f1, 5.6.0b1