Steps to reproduce:
1. Open new project
2. Select the context menu for the Inspector window at the top right
3. Click "Add Tab > Inspector"
4. Dock the second inspector window to the right so that the two inspector windows are parallel
5. Select some game Object, such as the "Main Camera"
6. Try to resize the inspector windows by grabbing the vertical line between them
7. More often than not, even when the cursor shows that you should be able to click and drag the separator, when you do click, nothing is dragged. You have to be very precisely in the middle to actually be able to grab and drag the window separator

Expected: If the cursor shows being able to drag, then when I click and drag, the windows should resize
Actual: The cursor shows being able to drag, but when you click and drag, nothing happens

Repro:
2019.3.0a5 (9aff892fb75b), 2019.3.0a4 (7368acd360f4), 2019.3.0a2 (fa7740529556), 2019.2.0b5 (e3a10156d6de), 2019.1.6f1 (76da8ff1860f), 2019.1.0b1 (83b3ba1f99df), 2019.1.0a11 (0f106840e011), 2019.1.0a10 (84f9b5a9b69f)

No Repro:
2018.4.1f1 (b7c424a951c0), 2019.1.0a8 (7ae038d0fb32), 2019.1.0a9 (0acd256790e8)

NOTE: You don't NEED to dock the two inspectors like in the repro steps for this to repro anyway, it's just easier to achieve the bug repro this way