GridSplitter does not work correctly, expands child outside window

* .NET Core Version: 3.0 Preview 8

* Windows version: Windows 10

* Does the bug reproduce also in WPF for .NET Framework 4.8?: Not sure

**Problem description:**

I have a window with a grid with 2 rows. Between the rows I have a grid splitter. I want the bottom row to start with an initial height, but allow user to expand it. Both rows can collapse to height 0 or expand to fill the entire window.

When I drag the grid splitter to the top edge and continue dragging the mouse (without releasing the button) the bottom row keeps expanding for ever. I expect the gird splitter to stop expanding the bottom row as soon as the splitter hits the top of the window.

When I drag the grid splitter down it works correctly.

**Actual behavior:**
When I drag the grid splitter to the top edge and continue dragging the mouse (without releasing the button) the bottom row keeps expanding for ever.

**Expected behavior:**
I expect the gird splitter to stop expanding the bottom row as soon as the splitter hits the top of the window.

**Minimal repro:**
Create a new WPF app with this xaml:
```

```

Run the application and move/size the application window so you can test dragging the grid splitter from its location to a point above the window.
Expand the grid splitter up and continue beyond window. The height is displayed in the text box. You will see it increasing.