Show members of selected relations even when their feature layer is otherwise hidden (close #6220)

modules/modes/select.js

@@ -233,7 +233,18 @@ export function modeSelect(context, selectedIDs) {
     mode.enter = function() {
         if (!checkSelectedIDs()) return;
 
-        context.features().forceVisible(selectedIDs);
+        var forceVisibleIDs = selectedIDs.slice();
+
+        selectedIDs.forEach(function(id) {
+            var entity = context.entity(id);
+            if (entity.type === 'relation') {
+                forceVisibleIDs = forceVisibleIDs.concat(entity.members.map(function(member) {
+                    return member.id;
+                }));
+            }
+        });
+
+        context.features().forceVisible(forceVisibleIDs);
 
         var operations = Object.values(Operations)
             .map(function(o) { return o(selectedIDs, context); })