webmail-client/frontend/src/components/Resizer.tsx

// A vertical drag handle for adjusting the width of the panel to its left.
export function Resizer({ onDelta }: { onDelta: (deltaX: number) => void }) {
  const onMouseDown = (e: React.MouseEvent) => {
    e.preventDefault();
    let lastX = e.clientX;
    const onMove = (ev: MouseEvent) => {
      onDelta(ev.clientX - lastX);
      lastX = ev.clientX;
    };
    const onUp = () => {
      document.removeEventListener("mousemove", onMove);
      document.removeEventListener("mouseup", onUp);
      document.body.style.userSelect = "";
      document.body.style.cursor = "";
    };
    document.addEventListener("mousemove", onMove);
    document.addEventListener("mouseup", onUp);
    document.body.style.userSelect = "none";
    document.body.style.cursor = "col-resize";
  };

  return (
    <div
      onMouseDown={onMouseDown}
      className="group relative z-20 -mx-0.5 w-1 shrink-0 cursor-col-resize"
      role="separator"
      aria-orientation="vertical"
    >
      <div className="absolute inset-y-0 left-1/2 w-px -translate-x-1/2 bg-transparent group-hover:bg-app-accent" />
      {/* Small pill grab handle in the middle for easier grabbing. */}
      <div className="absolute left-1/2 top-1/2 h-7 w-[5px] -translate-x-1/2 -translate-y-1/2 rounded-full border border-app-border bg-app-elevated group-hover:border-app-accent group-hover:bg-app-accent" />
    </div>
  );
}