To reproduce:
1. Open attached project
2. Open scene test_CanvasDropdownProblem if not opened
3. Hit play
4. Press show canvas and click on drop-down list
5. Press key q (this will call ShowCanvas(false); )
6. Now press show canvas again
7. Repeat 5,6 steps
8. Notice that drop-down list is disappearing

Expected : After re-enabling parent canvas drop-down list remains same as it was before
Actual: Drop-down list becomes transparent

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. Drop-down list is still working (it is possible to select an item).