Top window border missing on Windows with TitleBarStyle.hidden

[jagmit]

When using TitleBarStyle.hidden on Windows, the standard window border is missing on the top side. This breaks the visual consistency with native Windows apps, which usually have a thin accent-colored border around all four sides of the window. The issue is clearly visible in the example app included with this package.

Steps to Reproduce

1. Clone the window_manager repository and run the example app.
2. Set the window title bar style to TitleBarStyle.hidden.
3. Observe the window frame on Windows.

Expected Behavior
The window should have a consistent border on all four sides, including the top, as is standard on Windows.

Actual Behavior
The top border is missing, while the left, right, and bottom borders are present.

Environment

• OS: Windows 10 Pro, Version 22H2
• Flutter version: 3.29.3
• window_manager version: [fill in if known]

Screenshot

💡 To better highlight the missing top border, I changed the Windows accent color to red, which makes the borders easier to distinguish.
You can do the same by going to:
Right-click on Desktop > Personalize > Colors
and choosing a vivid accent color like red.