diff --git a/src/common.ts b/src/common.ts
index 9e85b514..8a55ff39 100644
--- a/src/common.ts
+++ b/src/common.ts
@@ -91,7 +91,7 @@ interface IConfig {
 
 interface IDriverWindow {
     readonly fullScreen: boolean;
-    readonly geometry: Rect;
+    readonly geometry: Readonly<Rect>;
     readonly id: string;
     readonly maximized: boolean;
     readonly shouldIgnore: boolean;
@@ -106,7 +106,7 @@ interface IDriverWindow {
 interface ISurface {
     readonly id: string;
     readonly ignore: boolean;
-    readonly workingArea: Rect;
+    readonly workingArea: Readonly<Rect>;
 
     next(): ISurface | null;
 }
diff --git a/src/util/rect.ts b/src/util/rect.ts
index 82a881fa..94ab0d5a 100644
--- a/src/util/rect.ts
+++ b/src/util/rect.ts
@@ -19,10 +19,13 @@
 // DEALINGS IN THE SOFTWARE.
 
 class Rect {
-    public readonly height: number;
-    public readonly width: number;
-    public readonly x: number;
-    public readonly y: number;
+    constructor(
+        public x: number,
+        public y: number,
+        public width: number,
+        public height: number,
+    ) {
+    }
 
     public get maxX(): number { return this.x + this.width; }
     public get maxY(): number { return this.y + this.height; }
@@ -34,11 +37,8 @@ class Rect {
         ];
     }
 
-    constructor(x: number, y: number, w: number, h: number) {
-        this.height = h;
-        this.width = w;
-        this.x = x;
-        this.y = y;
+    public clone(): Rect {
+        return new Rect(this.x, this.y, this.width, this.height);
     }
 
     public equals(other: Rect): boolean {
@@ -59,6 +59,14 @@ class Rect {
         );
     }
 
+    public gap_mut(left: number, right: number, top: number, bottom: number): this {
+        this.x += left;
+        this.y += top;
+        this.width -= (left + right);
+        this.height -= (top + bottom);
+        return this;
+    }
+
     public includes(other: Rect): boolean {
         return (
             this.x <= other.x &&