Reintroduced the Windows-specific fix

Note to self: never trust Windows users ever again when they report performance degradations. It’s never you. It’s always Windows.

Fixes #4
Fixes #10
Fixes #12
Fixes #14

src/main/java/dev/kir/cubeswithoutborders/mixin/WindowMixin.java

@@ -4,6 +4,7 @@
 import com.llamalad7.mixinextras.injector.wrapoperation.WrapOperation;
 import com.mojang.blaze3d.systems.RenderSystem;
 import dev.kir.cubeswithoutborders.client.BorderlessWindowState;
+import dev.kir.cubeswithoutborders.util.SystemUtil;
 import net.fabricmc.api.EnvType;
 import net.fabricmc.api.Environment;
 import net.minecraft.client.util.*;
@@ -119,10 +120,19 @@ private void updateBorderlessWindowRegion(CallbackInfo ci) {
         // which resets values of `width` and `height`.
         GLFW.glfwSetWindowAttrib(this.handle, GLFW.GLFW_DECORATED, GLFW.GLFW_FALSE);
 
+        // There's a bug that causes a fullscreen window to flicker when it loses focus.
+        // As far as I know, this is relevant for Windows and X11 desktops.
+        // Fuck X11 - it's a perpetually broken piece of legacy.
+        // However, we do need to implement a fix for Windows desktops, as they
+        // are not going anywhere in the foreseeable future (sadly enough).
+        // This "fix" involves not bringing a window into a "proper" fullscreen mode,
+        // but rather stretching it 1 pixel beyond the screen's supported resolution.
+        int heightOffset = SystemUtil.isWindows() ? 1 : 0;
+
         this.x = 0;
         this.y = 0;
         this.width = videoMode.getWidth();
-        this.height = videoMode.getHeight();
+        this.height = videoMode.getHeight() + heightOffset;
         GLFW.glfwSetWindowMonitor(this.handle, 0L, this.x, this.y, this.width, this.height, -1);
 
         this.currentBorderless = true;

src/main/java/dev/kir/cubeswithoutborders/util/SystemUtil.java

@@ -0,0 +1,15 @@
+package dev.kir.cubeswithoutborders.util;
+
+import java.util.Locale;
+
+public final class SystemUtil {
+    public static String getSystemName() {
+        return System.getProperty("os.name").toLowerCase(Locale.ROOT);
+    }
+
+    public static boolean isWindows() {
+        return SystemUtil.getSystemName().contains("win");
+    }
+
+    private SystemUtil() { }
+}