Pinch zoom implementation for PDF.js viewer

viewer.html

<!-- Goes into viewer.html just before ending </body> -->
<script>
    let pinchZoomEnabled = false;
    function enablePinchZoom(pdfViewer) {
        let startX = 0, startY = 0;
        let initialPinchDistance = 0;        
        let pinchScale = 1;    
        const viewer = document.getElementById("viewer");
        const container = document.getElementById("viewerContainer");
        const reset = () => { startX = startY = initialPinchDistance = 0; pinchScale = 1; };
        // Prevent native iOS page zoom
        //document.addEventListener("touchmove", (e) => { if (e.scale !== 1) { e.preventDefault(); } }, { passive: false });
        document.addEventListener("touchstart", (e) => {
            if (e.touches.length > 1) {
                startX = (e.touches[0].pageX + e.touches[1].pageX) / 2;
                startY = (e.touches[0].pageY + e.touches[1].pageY) / 2;
                initialPinchDistance = Math.hypot((e.touches[1].pageX - e.touches[0].pageX), (e.touches[1].pageY - e.touches[0].pageY));
            } else {
                initialPinchDistance = 0;
            }
        });
        document.addEventListener("touchmove", (e) => {
            if (initialPinchDistance <= 0 || e.touches.length < 2) { return; }
            if (e.scale !== 1) { e.preventDefault(); }
            const pinchDistance = Math.hypot((e.touches[1].pageX - e.touches[0].pageX), (e.touches[1].pageY - e.touches[0].pageY));
            const originX = startX + container.scrollLeft;
            const originY = startY + container.scrollTop;
            pinchScale = pinchDistance / initialPinchDistance;
            viewer.style.transform = `scale(${pinchScale})`;
            viewer.style.transformOrigin = `${originX}px ${originY}px`;
        }, { passive: false });
        document.addEventListener("touchend", (e) => {
            if (initialPinchDistance <= 0) { return; }
            viewer.style.transform = `none`;
            viewer.style.transformOrigin = `unset`;
            PDFViewerApplication.pdfViewer.currentScale *= pinchScale;
            const rect = container.getBoundingClientRect();
            const dx = startX - rect.left;
            const dy = startY - rect.top;
            container.scrollLeft += dx * (pinchScale - 1);
            container.scrollTop += dy * (pinchScale - 1);
            reset();
        });
    }
    document.addEventListener('webviewerloaded', () => {
        if (!pinchZoomEnabled) {
            pinchZoomEnabled = true;
            enablePinchZoom();
        }
    });
</script>