updated background itemtype. No content is allowed now.

dist/vis.css

@@ -232,6 +232,13 @@
   max-width: 100%;
   margin: 5px;
 }
+.vis.timeline .item.background .noContent {
+    position: absolute;
+    display: inline-block;
+    overflow: hidden;
+    max-width: 100%;
+    margin: 0px;
+}
 
 .vis.timeline .item.line {
   padding: 0;

lib/timeline/component/css/item.css

@@ -72,6 +72,13 @@
   max-width: 100%;
   margin: 5px;
 }
+.vis.timeline .item.background .noContent {
+    position: absolute;
+    display: inline-block;
+    overflow: hidden;
+    max-width: 100%;
+    margin: 0px;
+}
 
 .vis.timeline .item.line {
   padding: 0;

lib/timeline/component/item/BackgroundItem.js

@@ -31,6 +31,7 @@ function BackgroundItem (data, conversion, options) {
     }
   }
 
+  this.emptyContent = false;
   Item.call(this, data, conversion, options);
 }
 
@@ -58,13 +59,20 @@ BackgroundItem.prototype.redraw = function() {
     this.dom = {};
     dom = this.dom;
 
-      // background box
+    // background box
     dom.box = document.createElement('div');
     // className is updated in redraw()
 
     // contents box
     dom.content = document.createElement('div');
-    dom.content.className = 'content';
+    if (this.data.content === undefined) {
+      dom.content.className = 'noContent';
+      this.emptyContent = true;
+    }
+    else {
+      dom.content.className = 'content';
+      this.emptyContent = false;
+    }
     dom.box.appendChild(dom.content);
 
     // attach this item as attribute
@@ -91,6 +99,16 @@ BackgroundItem.prototype.redraw = function() {
   // - the item's data is changed
   // - the item is selected/deselected
   if (this.dirty) {
+    // only change content class when needed
+    if (this.data.content === undefined && this.emptyContent == false) {
+      dom.content.className = 'noContent';
+      this.emptyContent = true;
+    }
+    else if (this.data.content !== undefined && this.emptyContent == true) {
+      dom.content.className = 'content';
+      this.emptyContent = false;
+    }
+
     this._updateContents(this.dom.content);
     this._updateTitle(this.dom.content);
     this._updateDataAttributes(this.dom.content);

lib/timeline/component/item/Item.js

@@ -176,7 +176,9 @@ Item.prototype._updateContents = function (element) {
     element.innerHTML = content;
   }
   else {
-    throw new Error('Property "content" missing in item ' + this.data.id);
+    if (!(this.data.type == "background" && this.data.content === undefined)) {
+      throw new Error('Property "content" missing in item ' + this.data.id);
+    }
   }
 };