Change the default OverlayBox implementations.

Win32LayeredBox is preferable on Windows because of
WS_EX_TRANSPARENT.

Otherwise, the split window approach is simpler.

Author: madewokherd

xalia/Sdl/Win32WindowingSystem.cs

@@ -36,7 +36,7 @@ public override Task ShowKeyboardAsync()
 
         public override OverlayBox CreateOverlayBox()
         {
-            if (Utils.TryGetEnvironmentVariable("XALIA_LAYERED_BOX", out var value) &&
+            if (!Utils.TryGetEnvironmentVariable("XALIA_LAYERED_BOX", out var value) ||
                 value != "0")
                 return new Win32LayeredBox(this);
             return base.CreateOverlayBox();

xalia/Sdl/WindowingSystem.cs

@@ -48,7 +48,7 @@ public static WindowingSystem Instance
 
         public virtual OverlayBox CreateOverlayBox()
         {
-            if (Utils.TryGetEnvironmentVariable("XALIA_SPLIT_BOX", out var value) &&
+            if (!Utils.TryGetEnvironmentVariable("XALIA_SPLIT_BOX", out var value) ||
                 value != "0")
                 return new SplitOverlayBox(this);
             return new SdlOverlayBox(this);