Fix keyboard navigation in modal image viewer

javascript/imageviewer.js

@@ -151,6 +151,7 @@ function showGalleryImage() {
                     e.addEventListener('mousedown', function (evt) {
                         if(!opts.js_modal_lightbox || evt.button != 0) return;
                         modalZoomSet(gradioApp().getElementById('modalImage'), opts.js_modal_lightbox_initially_zoomed)
+                        evt.preventDefault()
                         showModal(evt)
                     }, true);
                 }