Steps to reproduce:

1. Open Unity on Windows;
2. Create new project;
3. Add Terrain gameobject (GameObject -> 3D Object -> Terrain);
4. Select Terrain gameobject in the Hierarchy view;
5. Go to Place Trees tab in Terrain Component;
6. Click Edit Trees -> Add Tree;
7. Click on main Editor window, while Add Tree popup window is active;
8. Minimize main Editor's window;
9. Expand main Editor's window;
10. Observe "Add Tree" popup window.

Actual result:
Popup window rendering disappears (see attached gif video actual.png screenshot). Also happens in every popup window through Terrain component:
- Add/Edit Terrain texture;
- Add/Edit Tree/Detail mesh;
- Mass Place Trees.

Regression introduced in 5.4.0.

Expected result:
Popup window should be rendered normally after minimizing/expanding main Editor's window (see attached expected.png screenshot).

Notes:
- Happens only on Windows (8 and 10 checked);
- Happens with DX11 and DX9 mode (checked);
- Popup window rendering appears when mouse hover or clicking on it;
- Doesn't reproduce in 5.3.0f4;
- May happen in other similar Editor windows.