Steps to reproduce:

1. Open attached project "776991.zip"
2. Open scene "repro"
3. In hierarchy, select Direction Light
4. Notice that it has Test Indent component. Try clicking on checkbox found in the inspector of component
5. Checkbox isn't clickable

Additional steps:
6. Open script TestIndentEditor.cs
7. Comment out line 20: EditorGUI.indentLevel++;

Note: This happens due to checkbox being indented and having low label width (=1). This happens with bool checkbox, input field of string or int works as expected even with low label width.

Workaround: Increase label width, around 15 is enough.

Reproduced with: 5.2.4f1, 5.3.3p2, 5.4.0b9