To reproduce:
1. Open a new Unity project
2. Create a new C# script and create a public array
3. Attach the script to any object
4. Extend the array to see the elements
5. Click on any element and press delete key

Expected result: element should be deleted.
Actual result: element does not get deleted.

Note: If you hold left mouse button and then press the delete key, element will get deleted successfully

Regression first introduced in: 2017.1.2p3
Reproduced on: 2017.1.2p3, 2017.2.0p3, 2017.3.0b11, 2018.1.0a5
Not reproduced on: 5.6.4p1, 2017.1.2p2