diff --git a/src/components/Output/custom-els/TwoUp/index.ts b/src/components/Output/custom-els/TwoUp/index.ts
index 53966c0a..0c76a5d2 100644
--- a/src/components/Output/custom-els/TwoUp/index.ts
+++ b/src/components/Output/custom-els/TwoUp/index.ts
@@ -18,6 +18,10 @@ export default class TwoUp extends HTMLElement {
    * The position of the split in pixels.
    */
   private _position = 0;
+  /**
+   * The position of the split in %.
+   */
+  private _relativePosition = 0.5;
   /**
    * The value of _position when the pointer went down.
    */
@@ -37,6 +41,14 @@ export default class TwoUp extends HTMLElement {
     new MutationObserver(() => this._childrenChange())
       .observe(this, { childList: true });
 
+    // Watch for element size changes.
+    if ('ResizeObserver' in window) {
+      new ResizeObserver(() => this._resetPosition())
+        .observe(this);
+    } else {
+      window.addEventListener('resize', () => this._resetPosition());
+    }
+
     // Watch for pointers on the handle.
     const pointerTracker: PointerTracker = new PointerTracker(this._handle, {
       start: (_, event) => {
@@ -57,7 +69,7 @@ export default class TwoUp extends HTMLElement {
 
   connectedCallback() {
     this._handle.innerHTML = `<div class="${styles.scrubber}">${
-      `<svg viewBox="0 0 20 10" fill="currentColor"><path d="M8 0v10L0 5zM12 0v10l8-5z"/></svg>`
+      '<svg viewBox="0 0 20 10" fill="currentColor"><path d="M8 0v10L0 5zM12 0v10l8-5z"/></svg>'
     }</div>`;
 
     this._childrenChange();
@@ -77,7 +89,8 @@ export default class TwoUp extends HTMLElement {
     // Set the initial position of the handle.
     requestAnimationFrame(() => {
       const bounds = this.getBoundingClientRect();
-      this._position = (this.orientation === 'vertical' ? bounds.height : bounds.width) / 2;
+      const dimensionAxis = this.orientation === 'vertical' ? 'height' : 'width';
+      this._position = bounds[dimensionAxis] * this._relativePosition;
       this._setPosition();
     });
   }
@@ -138,6 +151,7 @@ export default class TwoUp extends HTMLElement {
 
     // Clamp position to element bounds.
     this._position = Math.max(0, Math.min(this._position, bounds[dimensionAxis]));
+    this._relativePosition = this._position / bounds[dimensionAxis];
     this._setPosition();
   }
 
diff --git a/src/components/Output/custom-els/TwoUp/missing-types.d.ts b/src/components/Output/custom-els/TwoUp/missing-types.d.ts
index fde547bd..b3c129d5 100644
--- a/src/components/Output/custom-els/TwoUp/missing-types.d.ts
+++ b/src/components/Output/custom-els/TwoUp/missing-types.d.ts
@@ -19,3 +19,34 @@ declare namespace JSX {
     'two-up': TwoUpAttributes;
   }
 }
+
+interface DOMRectReadOnly {
+  readonly x: number;
+  readonly y: number;
+  readonly width: number;
+  readonly height: number;
+  readonly top: number;
+  readonly right: number;
+  readonly bottom: number;
+  readonly left: number;
+}
+
+interface ResizeObserverCallback {
+  (entries: ResizeObserverEntry[], observer: ResizeObserver): void;
+}
+
+interface ResizeObserverEntry {
+  readonly target: Element;
+  readonly contentRect: DOMRectReadOnly;
+}
+
+interface ResizeObserver {
+  observe(target: Element): void;
+  unobserve(target: Element): void;
+  disconnect(): void;
+}
+
+declare var ResizeObserver: {
+  prototype: ResizeObserver;
+  new(callback: ResizeObserverCallback): ResizeObserver;
+};