fix: collapse shadow padding when window touches a screen edge

[gadfly3173]

Closes #2716

[dwndland]

Tested on Endeavour OS)

It might be a little special, but it does not work in all the cases.
In my setup, two monitors are alligned vertically, so Monitor 2 is above Monitor 3.
And all monitors have a bottom panel configureed.
The shadow disappears at the bottom panel only if there is no screen below, so on my upper screen it still snaps with this shadow gap.

(Just as a side note. My setup is special, so this would not block anything IMHO)

[gadfly3173]

Avalonia can detect each physical monitor through Screen.Bounds, but Screen.WorkingArea is the usable area after panels and is not always available per monitor on Linux/X11.
With vertically stacked monitors, the upper monitor’s bottom panel is an internal edge of the virtual desktop, so the window may still retain its bottom shadow there:

┌──────────────────┐
│ Upper monitor    │
│                  │
│                  │
│ Upper panel      │
└──────────────────┘  ← internal desktop edge
┌──────────────────┐
│ Lower monitor    │
│                  │
│                  │
│ Lower panel      │
└──────────────────┘

This comes from src/Avalonia.X11/Screens/X11Screen.Providers.cs, where UpdateWorkArea() reads the root _NET_WORKAREA and sets WorkingArea = Bounds.Intersect(wa); src/Avalonia.Controls/Platform/ScreenHelper.cs then selects a screen based on the window’s intersecting area.

[dwndland]

I am on wayland, just as a side note :D
I think its right what you say. The working area does not support that something in the vertical middle of it is stealing space. I guess.

But interestingly, this is he only exception its seems.
My setup is following.
I have 3 monitors horizontally and a 4th in the center on top.
The screen edges left and right of the top monitor is seen right, just the lower panel not. I would have expected the working area would be have an imaginary monitor top left and top right, to get a full square :D