How to reproduce:
1. Open the user-submitted project
2. Open a Custom Editor window (Nementic -> Prefab Replacer)
3. Make the Prefab Replacer window small enough for the horizontal and vertical scrollbars to appear
4. Inspect the layout of the Prefab Replacer window in floating mode
5. Dock the Prefab Replacer window and resize it to a similar size as before
6. Inspect the layout of the Prefab Replacer window in docked mode

Expected result: Layout is the same for both docked and floating window modes
Actual result: Layout is different for docked and floating window modes

Reproducible with: 2018.4.5f1, 2019.1.13f1, 2019.2.0f1, 2019.3.0a11

Notes:
Issues are reproducible with standard Unity Editor windows as well. However, custom layouts in these windows make some issues invisible.
Window header box is cut on the left side while in floating mode (reproducible in 2018.4.5f1, 2019.1.13f1, 2019.2.0f1, 2019.3.0a11)
Scrollbars have a part of them cut off while in docked mode (reproducible in 2019.1.13f1, 2019.2.0f1)
Window content has a bigger gap from the top in floating mode (reproducible in 2018.4.5f1, 2019.1.13f1, 2019.2.0f1)
Window header has a larger gap from the left side while in docked mode (reproducible in 2019.1.13f1, 2019.2.0f1, 2019.3.0a11)