Top border persists with decorations disabled

[SpikeHD]

Describe the bug
When disabling decorations (in order to use a custom titlebar), a single-pixel-wide border on the top is still visible. The border is either grey (and quite hard to spot, usually) when Show the accent colour on the following surfaces -> Title bars and window borders is disabled, or the system accent colour when the setting is enabled.

This can be seen in Tao's decorations example. You may have to look closely at the top of the white window:

Steps To Reproduce

1. Optionally enable "Title bars and window borders" in Windows accent colour settings, in order to better see the effect
2. Create a Window with with_decorations(false)

Expected behavior
There should be no top border

Screenshots
In my project (for better visibility via the dark titlebar), using the custom titlebar (aka decorations off):

Using the system titlebar (aka decorations on):

Platform and Versions (please complete the following information):
OS: Windows 10 22H2 19045.5011
Rustc: 1.81.0

Additional Context
SpikeHD/Dorion#268

[amrbashir]

this should be fixed by #993 where all 4 sides will have borders which matches the native system behavior, if you want that disabled as well, you'll need to also disable shadows.