To reproduce:
1. Open attached project
2. Open scene test_CanvasDropdownProblem
3. Hit play
4. Press show canvas then open drop-down list
5. Press key Q two or more times
6. Press show canvas again and notice that drop-down list became invisible (but still you can click on its elements)

Expected: After calling this method twice and enabling canvas again drop-down list is visible
Actual: Drop-down list becomes invisible (still usable).

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. Even the drop-down list is invisible you can press on its elements.