Only warn about interactive HTML nodes; document HTML interactivity.

see numbas/Numbas#1120

docs/jme-reference.rst

@@ -492,6 +492,12 @@ Some extensions add new data types.
 
     See functions related to :ref:`jme-fns-html`.
 
+    .. warning::
+
+        Interactive HTML nodes can not be safely copied, so each HTML value should only be used once in a question.
+        You can mark an HTML node as non-interactive by adding the attribute ``data-interactive="false"`` to it.
+        Elements created using the built-in HTML functions are automatically marked as non-interactive.
+
 .. data:: expression
 
     A JME sub-expression.

editor/static/js/numbas/numbas-runtime.js

@@ -3564,6 +3564,9 @@ var TBool = types.TBool = function(b) {
 jme.registerType(TBool,'boolean');
 
 /** HTML DOM element.
+ *
+ * If the element has the attribute `data-interactive="false"` then it can be safely copied and embedded multiple times.
+ * If the attribute is not present or has any other value, then it's assumed that it can't be safely copied.
  *
  * @memberof Numbas.jme.types
  * @augments Numbas.jme.token
@@ -3589,6 +3592,11 @@ var THTML = types.THTML = function(html) {
     this.value = Array.from(elem.childNodes);
     this.html = elem.innerHTML;
 }
+THTML.prototype = {
+    isInteractive: function() {
+        return this.value.some(e => e.getAttribute('data-interactive') !== 'false');
+    }
+}
 jme.registerType(THTML,'html');
 
 /** List of elements of any data type.
@@ -6314,7 +6322,9 @@ newBuiltin('html',[TString],THTML,null, {
         container.innerHTML = args[0].value;
         var subber = new jme.variables.DOMcontentsubber(scope);
         subber.subvars(container);
-        return new THTML(Array.from(container.childNodes));
+        var nodes = Array.from(container.childNodes);
+        nodes.forEach(node => node.setAttribute('data-interactive', 'false'));
+        return new THTML(nodes);
     }
 });
 newBuiltin('isnonemptyhtml',[TString],TBool,function(html) {
@@ -6335,6 +6345,7 @@ newBuiltin('image',[TString, '[number]', '[number]'],THTML,null, {
         }
         var subber = new jme.variables.DOMcontentsubber(scope);
         var element = subber.subvars(img);
+        element.setAttribute('data-interactive', 'false');
         return new THTML(element);
     }
 });
@@ -6810,6 +6821,7 @@ newBuiltin('scientificnumberhtml', [TDecimal], THTML, function(n) {
     var bits = math.parseScientific(n.re.toExponential());
     var s = document.createElement('span');
     s.innerHTML = math.niceRealNumber(bits.significand)+' × 10<sup>'+bits.exponent+'</sup>';
+    s.setAttribute('data-interactive', 'false');
     return s;
 });
 newBuiltin('scientificnumberhtml', [TNum], THTML, function(n) {
@@ -6819,6 +6831,7 @@ newBuiltin('scientificnumberhtml', [TNum], THTML, function(n) {
     var bits = math.parseScientific(math.niceRealNumber(n,{style:'scientific', scientificStyle:'plain'}));
     var s = document.createElement('span');
     s.innerHTML = math.niceRealNumber(bits.significand)+' × 10<sup>'+bits.exponent+'</sup>';
+    s.setAttribute('data-interactive', 'false');
     return s;
 });
 
@@ -8378,6 +8391,7 @@ newBuiltin('table',[TList,TList],THTML, null, {
                 row.appendChild(td);
             }
         }
+        table.setAttribute('data-interactive','false');
         return new THTML(table);
     }
 });
@@ -8396,6 +8410,7 @@ newBuiltin('table',[TList],THTML, null, {
                 row.appendChild(td);
             }
         }
+        table.setAttribute('data-interactive','false');
         return new THTML(table);
     }
 });
@@ -14110,6 +14125,9 @@ jme.variables = /** @lends Numbas.jme.variables */ {
         function doToken(token) {
             if(jme.isType(token,'html')) {
                 token = jme.castToType(token,'html');
+                if(!token.isInteractive()) {
+                    return token.value.map(e => e.cloneNode(true));
+                }
                 if(token.value.numbas_embedded) {
                     throw(new Numbas.Error('jme.subvars.html inserted twice'))
                 }

editor/static/js/question/edit.js

@@ -2431,12 +2431,12 @@ $(document).ready(function() {
             return val.type;
         },this);
 
-        this.isHTML = ko.pureComputed(function() {
+        this.isInteractiveHTML = ko.pureComputed(function() {
             var val = this.value();
             if(!val || this.error()) {
                 return false;
             }
-            return Numbas.jme.isType(val,'html');
+            return Numbas.jme.isType(val,'html') && val.isInteractive();
         },this);
 
         this.thisLocked = ko.observable(false);

editor/templates/question/tabs/variables.html

@@ -251,8 +251,8 @@ <h5>Suggestions</h5>
                                 </li>
                             </ul>
                         </div>
-                        <div class="html-type-warning value-error alert alert-danger" data-bind="visible: isHTML">
-                            <p>This variable is an HTML node. HTML nodes can not be relied upon to work correctly when resuming a session - for example, attached event callbacks will be lost, and mathematical notation will likely also break.</p>
+                        <div class="html-type-warning value-error alert alert-danger" data-bind="visible: isInteractiveHTML">
+                            <p>This variable is an interactive HTML node. Interactive HTML nodes can not be relied upon to work correctly when resuming a session - for example, attached event callbacks will be lost, and mathematical notation will likely also break.</p>
                             <p>If this causes problems, try to create HTML nodes where you use them in content areas, instead of storing them in variables.</p>
                         </div>
                     </div>