To reproduce:
1. Open project attached by the user
2. Open scene test_CanvasDropdownProblem (if not opened)
3. Hit play
4. Press show canvas
5. Click on the dropdown to open it
6. Press the "Q" key on the keyboard to hide the parent canvas of the dropdown
7. Click on Show Canvas button again
8. Press on dropdown and notice that it is unclickable

Expected: After re-enabling parent canvas via script, drop-down list will be still clickable
Actual: Drop-down list becomes unclickable.

Reproduced with 5.5.0f3, 5.6.0f1, 5.6.0p4, 5.6.1f1, 2017.1.0b4

Note : Re-enabling canvas in inspector works properly.

Workaround : Delay between disabling drop-down list and disabling parent canvas. (included in script attached by the user)