How to reproduce:
1. Open user's attached "tooling-test.zip" project
2. Go to Test -> Windows Testing
3. Click the dropdown arrow in the center of the button
4. Click the edge of the dropdown arrow button
5. Open "WindowTesting.cs" script
6. Change the line 31 from "EditorGUI.indentLevel++;" to "EditorGUI.indentLevel = 0;"
7. Repeat 2-4 steps

Expected result: Properties panel opens when the dropdown arrow button is pressed on the center of it with both EditorGUI.indentLevel set to 0 or 1 and there is no need to press on the edge of the dropdown arrow button for the properties panel to open
Actual result: If the EditorGUI.indentLevel is higher than 0, the dropdown arrow button becomes only clickable by the edge of the button (step 4). After the 6th step, the properties panel opens when clicking the center of the dropdown arrow button

Reproducible with: 2019.4.4f1, 2020.1.0b16, 2020.2.0a18
Could not test with: 2018.4.25f1 (ParticleSystem.MinMaxGradient doesn't have color selection)