As @tom-chadwin mentioned it's not currently supported but you should be able to implement a basic version by making the following changes to your exported OL3 code:
Replace the onSingleClick
function in resources/qgis2web.js
with:
var onSingleClick = function(evt) {
if (doHover) {
return;
}
var pixel = map.getEventPixel(evt.originalEvent);
var coord = evt.coordinate;
var popupField;
var popupText = '';
var currentFeature;
var currentFeatureKeys;
map.forEachFeatureAtPixel(pixel, function(feature, layer) {
currentFeature = feature;
currentFeatureKeys = currentFeature.getKeys();
var field = getPopupFields(layersList, layer);
if (field == NO_POPUP) {
} else {
popupText += '<h2>' + layer.get('title') + '</h2>';
if (field == ALL_FIELDS) {
popupText += '<table>';
for (var i=0; i<currentFeatureKeys.length; i++) {
if (currentFeatureKeys[i] != 'geometry') {
popupField = '<th>' + currentFeatureKeys[i] + ':</th><td>'
popupField += (currentFeature.get(currentFeatureKeys[i]) != null ? Autolinker.link(String(currentFeature.get(currentFeatureKeys[i]))) + '</td>' : '');
popupText += '<tr>' + popupField + '</tr>';
}
}
popupText += '</table>';
} else {
var value = feature.get(field);
if (value) {
popupText += '<strong>' + field + ':</strong> '+ value;
}
}
}
});
if (popupText) {
overlayPopup.setPosition(coord);
content.innerHTML = popupText;
container.style.display = 'block';
} else {
container.style.display = 'none';
closer.blur();
}
};
Add some CSS for the popup in resources/qgis2web.css
:
div#popup-content {
max-height: 200px;
overflow-y: auto;
overflow-x: hidden;
padding-right: 20px;
}
I'll probably look at proposing a more complete implementation to qgis2web when I've got some time.