diff --git a/etc/layout_viewer/viewer.html b/etc/layout_viewer/viewer.html
index 76b85641e38..2deded56f8e 100644
--- a/etc/layout_viewer/viewer.html
+++ b/etc/layout_viewer/viewer.html
@@ -1,339 +1,276 @@
 <!DOCTYPE html>
 <html lang="en">
-  <head>
-    <meta charset="utf-8" />
-    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
-    <meta name="viewport" content="width=device-width, initial-scale=1" />
-    <title>Servo Layout Debugger</title>
+    <head>
+        <meta charset="utf-8">
+        <meta http-equiv="X-UA-Compatible" content="IE=edge">
+        <meta name="viewport" content="width=device-width, initial-scale=1">
+        <title>Servo Layout Debugger</title>
 
-    <!-- Bootstrap -->
-    <link href="css/bootstrap.min.css" rel="stylesheet" />
+        <!-- Bootstrap -->
+        <link href="css/bootstrap.min.css" rel="stylesheet">
 
-    <!-- Treeview -->
-    <link href="css/bootstrap-treeview.min.css" rel="stylesheet" />
+        <!-- Treeview -->
+        <link href="css/bootstrap-treeview.min.css" rel="stylesheet">
 
-    <!-- JSDiffPatch -->
-    <link href="css/formatters/html.css" rel="stylesheet" />
+        <!-- JSDiffPatch -->
+        <link href="css/formatters/html.css" rel="stylesheet">
 
-    <!-- Custom -->
-    <link href="css/main.css" rel="stylesheet" />
+        <!-- Custom -->
+        <link href="css/main.css" rel="stylesheet">
 
-    <!--[if lt IE 9]>
-      <script src="https://oss.maxcdn.com/html5shiv/3.7.2/html5shiv.min.js"></script>
-      <script src="https://oss.maxcdn.com/respond/1.4.2/respond.min.js"></script>
-    <![endif]-->
-  </head>
-  <body>
-    <div class="container" role="main">
-      <div class="row">
-        <div class="col-sm-12">
-          <h1>Servo Layout Viewer</h1>
-          <p>
-            Check the
-            <a
-              href="https://github.com/servo/servo/blob/master/etc/layout_viewer/README.md"
-              >README</a
-            >
-            for instructions.
-          </p>
-        </div>
-      </div>
-      <div class="row">
-        <div class="col-sm-4">
-          <div class="row">
-            <div class="col-sm-12">
-              <div class="well">
-                <input type="file" />
-              </div>
-            </div>
-          </div>
-          <div class="row">
-            <div class="col-sm-12">
-              <div id="trace-tree"></div>
-            </div>
-          </div>
-          <div class="row">
-            <div class="col-sm-12">
-              <ul id="trace-list" class="list-group"></ul>
-            </div>
-          </div>
-        </div>
-        <div class="col-sm-8">
-          <div class="row">
-            <div class="col-sm-12">
-              <div class="panel panel-default">
-                <div class="panel-heading">
-                  Box Tree
-                  <a
-                    id="box-tree-collapse"
-                    class="tree-collapse"
-                    data-collapsed="0"
-                  ></a>
+        <!--[if lt IE 9]>
+          <script src="https://oss.maxcdn.com/html5shiv/3.7.2/html5shiv.min.js"></script>
+          <script src="https://oss.maxcdn.com/respond/1.4.2/respond.min.js"></script>
+        <![endif]-->
+    </head>
+    <body>
+        <div class="container" role="main">
+            <div class="row">
+                <div class="col-sm-12">
+                  <h1> Servo Layout Viewer </h1>
+                  <p> Check the <a href="https://github.com/servo/servo/blob/master/etc/layout_viewer/README.md">README</a> for instructions.</p>
                 </div>
-                <div class="panel-body" id="box-tree"></div>
-              </div>
             </div>
-            <div class="col-sm-12">
-              <div id="box-diffs"></div>
-            </div>
-          </div>
-          <div class="row">
-            <div class="col-sm-12">
-              <div class="panel panel-default">
-                <div class="panel-heading">
-                  Fragment Tree
-                  <a
-                    id="fragment-tree-collapse"
-                    class="tree-collapse"
-                    data-collapsed="0"
-                  ></a>
+            <div class="row">
+                <div class="col-sm-4">
+                    <div class="row">
+                        <div class="col-sm-12">
+                            <div class="well">
+                                <input type=file>
+                            </div>
+                        </div>
+                    </div>
+                    <div class="row">
+                        <div class="col-sm-12">
+                            <div id="trace-tree">
+                            </div>
+                        </div>
+                    </div>
+                    <div class="row">
+                        <div class="col-sm-12">
+                            <ul id="trace-list" class="list-group">
+                            </ul>
+                        </div>
+                    </div>
+                </div>
+                <div class="col-sm-8">
+                    <div class="row">
+                        <div class="col-sm-12">
+                            <div class='panel panel-default'>
+                                <div class='panel-heading'>Flow Tree</div>
+                                <div class='panel-body' id="flow-tree"></div>
+                            </div>
+                        </div>
+                        <div class="col-sm-12">
+                            <div id="flow-diffs"></div>
+                        </div>
+                    </div>
                 </div>
-                <div class="panel-body" id="fragment-tree"></div>
-              </div>
             </div>
-            <div class="col-sm-12">
-              <div id="fragment-diffs"></div>
+            <div id="toolbar">
+              <a href="#" id="prev_trace">< Prev step</a>
+              |
+              <a href="#" id="next_trace">Next step ></a>
+              <br>
+              <input type="checkbox" name="show_unchanged" id="show_unchanged" />
+              <label for="show_unchanged">Show unchanged code</label>
+              <br>
+              <a href="#top">Back to top</a>
             </div>
-          </div>
         </div>
-      </div>
-    </div>
 
-    <!-- jQuery -->
-    <script src="js/jquery.min.js"></script>
-    <!-- Bootstrap -->
-    <script src="js/bootstrap.min.js"></script>
-    <!-- Treeview -->
-    <script src="js/bootstrap-treeview.min.js"></script>
-    <!-- JSDiffPatch -->
-    <script src="js/bundle.min.js"></script>
-    <script src="js/formatters.min.js"></script>
+        <!-- jQuery -->
+        <script src="js/jquery.min.js"></script>
+        <!-- Bootstrap -->
+        <script src="js/bootstrap.min.js"></script>
+        <!-- Treeview -->
+        <script src="js/bootstrap-treeview.min.js"></script>
+        <!-- JSDiffPatch -->
+        <script src="js/bundle.min.js"></script>
+        <script src="js/formatters.min.js"></script>
 
-    <script>
-      function get_inner_boxes(box) {
-        const box_type = Object.keys(box)[0];
-        switch (box_type) {
-          case "BlockLevelBoxes":
-            return box.BlockLevelBoxes;
-          case "InlineFormattingContext":
-            return box.InlineFormattingContext.inline_level_boxes;
-          case "InlineBox":
-            return box.InlineBox.children;
-          case "SameFormattingContextBlock":
-          case "Independent":
-          case "Flow":
-          case "OutOfFlowAbsolutelyPositionedBox":
-          case "OutOfFlowFloatBox":
-          case "Atomic":
-            return box[box_type].contents;
-        }
-        return null;
-      }
-
-      function box_tree_from_container(container) {
-        const box_type = Object.keys(container)[0];
-        let inner_boxes = get_inner_boxes(container);
-        let nodes = [];
-        let text = box_type;
-        if (Array.isArray(inner_boxes)) {
-          if (!inner_boxes.length) {
-            nodes = null;
-          } else {
-            for (let box in inner_boxes) {
-              nodes.push(box_tree_from_container(inner_boxes[box]));
-            }
-          }
-        } else if (inner_boxes != null) {
-          nodes.push(box_tree_from_container(inner_boxes));
-        } else {
-          if (box_type == "TextRun") {
-            text += ` (${container.TextRun.text})`;
-          }
-          nodes = null;
-        }
-
-        let info;
-        if (
-          box_type != "BlockLevelBoxes" &&
-          box_type != "InlineFormattingContext"
-        ) {
-          info = Object.assign({}, Object.values(container)[0]);
-          delete info.children;
-          delete info.contents;
-          delete info.tag;
-        }
-
-        return {
-          text,
-          nodes,
-          info
-        };
-      }
-
-      function box_tree_from_bfc(bfc) {
-        const { contains_floats, contents } = bfc;
-        let block_container = Object.values(contents)[0];
-        return {
-          text: "BlockFormattingContext",
-          info: {
-            contains_floats
-          },
-          nodes: [box_tree_from_container(contents)]
-        };
-      }
-
-      function create_fragment_tree(root) {
-        let fragment = Object.values(root)[0];
-        let node = {
-          text: Object.keys(root)[0],
-          id: fragment.debug_id,
-          href: "#diff-" + fragment.debug_id
-        };
-
-        if (fragment.children) {
-          let children = [];
-          for (let i = 0; i < fragment.children.length; ++i) {
-            children.push(create_fragment_tree(fragment.children[i]));
-          }
-
-          if (children.length > 0) {
-            node.nodes = children;
-          }
-        }
-
-        node.info = Object.assign({}, fragment);
-        delete node.info.children;
-        delete node.info.debug_id;
-
-        return node;
-      }
-
-      function flatten_trace(trace_node) {
-        const fragment_tree_root = Object.values(
-          trace_node.fragment_tree.children
-        )[0];
-        return {
-          fragment_tree: create_fragment_tree(fragment_tree_root),
-          box_tree: box_tree_from_bfc(trace_node.box_tree)
-        };
-      }
-
-      function create_trace_tree_node(trace_node) {
-        const trace = flatten_trace(trace_node.pre);
-
-        let tree_node = {
-          text: trace_node.name,
-          icon: "dummy",
-          box_tree: trace.box_tree,
-          fragment_tree: trace.fragment_tree
-        };
-
-        let node = Object.values(trace_node)[0];
-        if (node.children) {
-          let children = [];
-          for (let i = 0; i < node.children.length; ++i) {
-            children.push(create_trace_tree_node(node.children[i]));
-          }
-
-          if (children.length > 0) {
-            tree_node.nodes = children;
-          }
-        }
-
-        return tree_node;
-      }
-
-      function new_data_loaded(data) {
-        jsondiffpatch.formatters.html.hideUnchanged();
-
-        let node_color_hash = {};
-        let tree = [create_trace_tree_node(data)];
-        $("#trace-tree").treeview({ data: tree, levels: 3 });
-        $("#trace-tree").on("nodeSelected", function(event, node) {
-          $("#fragment-diffs").empty();
-          $("#trace-tree")
-            .treeview(true)
-            .revealNode(node);
-
-          const on_tree_node_selected = tree => (event, data) => {
-            $(`#${tree}-diffs`).empty();
-            if (!data.info) return;
-            // XXX(ferjm) no diff for now.
-            const delta = jsondiffpatch
-              .create({
-                objectHash: function(obj) {
-                  return JSON.stringify(obj);
+        <script>
+            function get_base(trace_node) {
+                if (typeof(trace_node.data.base) == "undefined" && typeof(trace_node.data.block_flow) != "undefined") {
+                  return trace_node.data.block_flow.base;
                 }
-              })
-              .diff({}, data.info);
+                else {
+                  return trace_node.data.base;
+                }
+            }
 
-            const json = jsondiffpatch.formatters.html.format(delta, data.info);
+            function create_flow_tree(trace_node) {
+                var base = get_base(trace_node);
 
-            $(`#${tree}-diffs`).append(
-              "<div class='panel panel-default'><div class='panel-heading'>" +
-                data.text +
-                "</div><div class='panel-body'>" +
-                json +
-                "</div></div>"
-            );
-          };
+                var node = {
+                    text: trace_node.class + " (" + base.id + ")",
+                    id: base.id,
+                    icon: "dummy",
+                    href: "#diff-" + base.id
+                };
 
-          const on_fragment_tree_node_selected = on_tree_node_selected(
-            "fragment"
-          );
-          const on_box_tree_node_selected = on_tree_node_selected("box");
+                var children = [];
+                for (var i=0 ; i < base.children.length ; ++i) {
+                    children.push(create_flow_tree(base.children[i]));
+                }
 
-          $("#fragment-tree").treeview({
-            data: [node.fragment_tree],
-            levels: 100,
-            enableLinks: false,
-            emptyIcon: "glyphicon glyphicon-unchecked hidden-glyphicon",
-            onNodeSelected: on_fragment_tree_node_selected
-          });
+                if (children.length > 0) {
+                    node.nodes = children;
+                }
 
-          $("#box-tree").treeview({
-            data: [node.box_tree],
-            levels: 100,
-            enableLinks: false,
-            emptyIcon: "glyphicon glyphicon-unchecked hidden-glyphicon",
-            onNodeSelected: on_box_tree_node_selected
-          });
+                return node;
+            }
 
-          ["fragment", "box"].forEach(key => {
-            const collapsable = $(`#${key}-tree-collapse`);
-            collapsable.html("Collapse all").on("click", () => {
-              const collapsed = collapsable.data("collapsed");
-              if (collapsed == 0) {
-                $(`#${key}-tree`).treeview("collapseAll");
-              } else {
-                $(`#${key}-tree`).treeview("expandAll");
-              }
-              collapsable.html(collapsed == 0 ? "Expand all" : "Collapse all");
-              collapsable.data("collapsed", collapsed == 0 ? 1 : 0);
+            function create_flow_hash(trace_node, flow_hash) {
+                var base = get_base(trace_node);
+                flow_hash[base.id] = trace_node;
+
+                for (var i=0 ; i < base.children.length ; ++i) {
+                    create_flow_hash(base.children[i], flow_hash);
+                }
+
+                delete base.children;
+            }
+
+            function flatten_trace(trace_node) {
+                var flow_tree = create_flow_tree(trace_node.children[0]);
+
+                var flow_hash = {};
+                create_flow_hash(trace_node.children[0], flow_hash);
+
+                return {
+                    tree: flow_tree,
+                    flows: flow_hash,
+                }
+            }
+
+            function create_tree_node(trace_node) {
+                var pre_trace = flatten_trace(trace_node.pre);
+                var post_trace = flatten_trace(trace_node.post);
+
+                var tree_node = {
+                    text: trace_node.name,
+                    icon: "dummy",
+                    flow_tree: pre_trace.tree,      // assume pre/post trace always have same tree!
+                    pre: pre_trace.flows,
+                    post: post_trace.flows,
+                };
+
+                var children = [];
+
+                for (var i=0 ; i < trace_node.children.length ; ++i) {
+                    children.push(create_tree_node(trace_node.children[i]));
+                }
+
+                if (children.length > 0) {
+                    tree_node.nodes = children;
+                }
+
+                return tree_node;
+            }
+
+            function update_flow_tree_bgcolor(flow_tree_node, node_color_hash) {
+                flow_tree_node.backColor = node_color_hash[flow_tree_node.id];
+                if (flow_tree_node.nodes !== undefined) {
+                    for (var i=0 ; i < flow_tree_node.nodes.length ; ++i) {
+                        update_flow_tree_bgcolor(flow_tree_node.nodes[i], node_color_hash)
+                    }
+                }
+            }
+
+            function new_data_loaded(data) {
+                jsondiffpatch.formatters.html.hideUnchanged();
+
+                var node_color_hash = {};
+                var tree = [ create_tree_node(data) ];
+                $('#trace-tree').treeview({data: tree, levels: 3});
+                $('#trace-tree').on('nodeSelected', function(event, node) {
+                    $("#flow-diffs").empty();
+                    $('#trace-tree').treeview(true).revealNode(node);
+
+                    for (var key in node.pre) {
+                        var flow_left = node.pre[key];
+                        var flow_right = node.post[key];
+
+                        var delta = jsondiffpatch.create({
+                            objectHash: function(obj) {
+                                if (obj.data !== undefined && obj.data.base !== undefined) {
+                                    return obj.data.base.id;
+                                }
+                                if (obj.id !== undefined) {
+                                    return obj.id;
+                                }
+                                if (obj.index !== undefined) {
+                                    // FlexItem and FlexLine
+                                    return obj.index;
+                                }
+                                return JSON.stringify(obj);
+                            }
+                        }).diff(flow_left, flow_right);
+
+                        if (delta !== undefined) {
+                            var diff_id = "diff-" + key;
+                            $("#flow-diffs").append(
+                              "<div class='panel panel-default' id='" +
+                              diff_id +
+                              "'><div class='panel-heading'>" +
+                              flow_left.class + " (" + key + ")" +
+                              "</div><div class='panel-body'></div></div>");
+
+                            document.getElementById(diff_id).getElementsByClassName('panel-body')[0].innerHTML =
+                                jsondiffpatch.formatters.html.format(delta, flow_left);
+                            node_color_hash[key] = "rgba(255, 0, 0, 0.7)";
+                        } else {
+                            node_color_hash[key] = "rgb(212, 248, 212)";
+                        }
+                    }
+
+                    update_flow_tree_bgcolor(node.flow_tree, node_color_hash);
+                    $('#flow-tree').treeview({data: [node.flow_tree], levels: 100, enableLinks: true, emptyIcon: "glyphicon glyphicon-unchecked hidden-glyphicon"});
+                });
+
+                $('#trace-tree').treeview(true).selectNode(0);
+            }
+
+            function register_toggle_unchanaged_code_handler() {
+              var show_unchange_box = document.getElementById("show_unchanged");
+              show_unchange_box.addEventListener("change", function(evt){
+                  jsondiffpatch.formatters.html.showUnchanged(show_unchange_box.checked, null, 800);
+              });
+            }
+
+            function register_prev_next_trace_node() {
+              var prev_btn = document.getElementById("prev_trace");
+              var next_btn = document.getElementById("next_trace");
+              prev_btn.addEventListener("click", function(evt){
+                var node_id = $("#trace-tree").treeview(true).getSelected()[0].nodeId;
+                // We deliberatly choose to ignore the node_id out of bound case,
+                // since it won't break the UI usability
+                $("#trace-tree").treeview(true).selectNode(node_id - 1);
+              });
+              next_btn.addEventListener("click", function(evt){
+                var node_id = $("#trace-tree").treeview(true).getSelected()[0].nodeId;
+                $("#trace-tree").treeview(true).selectNode(node_id + 1);
+              });
+            }
+
+            $( document ).ready(function() {
+                var upload = document.getElementsByTagName('input')[0];
+                upload.onchange = function (e) {
+                    e.preventDefault();
+
+                    var file = upload.files[0],
+                    reader = new FileReader();
+                    reader.onload = function (event) {
+                        new_data_loaded(JSON.parse(event.target.result));
+                    };
+
+                    reader.readAsText(file);
+                    return false;
+                };
+                register_toggle_unchanaged_code_handler();
+                register_prev_next_trace_node();
             });
-          });
-        });
-
-        $("#trace-tree")
-          .treeview(true)
-          .selectNode(0);
-      }
-
-      $(document).ready(function() {
-        let upload = document.getElementsByTagName("input")[0];
-        upload.onchange = function(e) {
-          e.preventDefault();
-
-          let file = upload.files[0],
-            reader = new FileReader();
-          reader.onload = function(event) {
-            new_data_loaded(JSON.parse(event.target.result));
-          };
-
-          reader.readAsText(file);
-          return false;
-        };
-      });
-    </script>
-  </body>
+        </script>
+    </body>
 </html>
diff --git a/etc/layout_viewer/viewer_2020.html b/etc/layout_viewer/viewer_2020.html
new file mode 100644
index 00000000000..76b85641e38
--- /dev/null
+++ b/etc/layout_viewer/viewer_2020.html
@@ -0,0 +1,339 @@
+<!DOCTYPE html>
+<html lang="en">
+  <head>
+    <meta charset="utf-8" />
+    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
+    <meta name="viewport" content="width=device-width, initial-scale=1" />
+    <title>Servo Layout Debugger</title>
+
+    <!-- Bootstrap -->
+    <link href="css/bootstrap.min.css" rel="stylesheet" />
+
+    <!-- Treeview -->
+    <link href="css/bootstrap-treeview.min.css" rel="stylesheet" />
+
+    <!-- JSDiffPatch -->
+    <link href="css/formatters/html.css" rel="stylesheet" />
+
+    <!-- Custom -->
+    <link href="css/main.css" rel="stylesheet" />
+
+    <!--[if lt IE 9]>
+      <script src="https://oss.maxcdn.com/html5shiv/3.7.2/html5shiv.min.js"></script>
+      <script src="https://oss.maxcdn.com/respond/1.4.2/respond.min.js"></script>
+    <![endif]-->
+  </head>
+  <body>
+    <div class="container" role="main">
+      <div class="row">
+        <div class="col-sm-12">
+          <h1>Servo Layout Viewer</h1>
+          <p>
+            Check the
+            <a
+              href="https://github.com/servo/servo/blob/master/etc/layout_viewer/README.md"
+              >README</a
+            >
+            for instructions.
+          </p>
+        </div>
+      </div>
+      <div class="row">
+        <div class="col-sm-4">
+          <div class="row">
+            <div class="col-sm-12">
+              <div class="well">
+                <input type="file" />
+              </div>
+            </div>
+          </div>
+          <div class="row">
+            <div class="col-sm-12">
+              <div id="trace-tree"></div>
+            </div>
+          </div>
+          <div class="row">
+            <div class="col-sm-12">
+              <ul id="trace-list" class="list-group"></ul>
+            </div>
+          </div>
+        </div>
+        <div class="col-sm-8">
+          <div class="row">
+            <div class="col-sm-12">
+              <div class="panel panel-default">
+                <div class="panel-heading">
+                  Box Tree
+                  <a
+                    id="box-tree-collapse"
+                    class="tree-collapse"
+                    data-collapsed="0"
+                  ></a>
+                </div>
+                <div class="panel-body" id="box-tree"></div>
+              </div>
+            </div>
+            <div class="col-sm-12">
+              <div id="box-diffs"></div>
+            </div>
+          </div>
+          <div class="row">
+            <div class="col-sm-12">
+              <div class="panel panel-default">
+                <div class="panel-heading">
+                  Fragment Tree
+                  <a
+                    id="fragment-tree-collapse"
+                    class="tree-collapse"
+                    data-collapsed="0"
+                  ></a>
+                </div>
+                <div class="panel-body" id="fragment-tree"></div>
+              </div>
+            </div>
+            <div class="col-sm-12">
+              <div id="fragment-diffs"></div>
+            </div>
+          </div>
+        </div>
+      </div>
+    </div>
+
+    <!-- jQuery -->
+    <script src="js/jquery.min.js"></script>
+    <!-- Bootstrap -->
+    <script src="js/bootstrap.min.js"></script>
+    <!-- Treeview -->
+    <script src="js/bootstrap-treeview.min.js"></script>
+    <!-- JSDiffPatch -->
+    <script src="js/bundle.min.js"></script>
+    <script src="js/formatters.min.js"></script>
+
+    <script>
+      function get_inner_boxes(box) {
+        const box_type = Object.keys(box)[0];
+        switch (box_type) {
+          case "BlockLevelBoxes":
+            return box.BlockLevelBoxes;
+          case "InlineFormattingContext":
+            return box.InlineFormattingContext.inline_level_boxes;
+          case "InlineBox":
+            return box.InlineBox.children;
+          case "SameFormattingContextBlock":
+          case "Independent":
+          case "Flow":
+          case "OutOfFlowAbsolutelyPositionedBox":
+          case "OutOfFlowFloatBox":
+          case "Atomic":
+            return box[box_type].contents;
+        }
+        return null;
+      }
+
+      function box_tree_from_container(container) {
+        const box_type = Object.keys(container)[0];
+        let inner_boxes = get_inner_boxes(container);
+        let nodes = [];
+        let text = box_type;
+        if (Array.isArray(inner_boxes)) {
+          if (!inner_boxes.length) {
+            nodes = null;
+          } else {
+            for (let box in inner_boxes) {
+              nodes.push(box_tree_from_container(inner_boxes[box]));
+            }
+          }
+        } else if (inner_boxes != null) {
+          nodes.push(box_tree_from_container(inner_boxes));
+        } else {
+          if (box_type == "TextRun") {
+            text += ` (${container.TextRun.text})`;
+          }
+          nodes = null;
+        }
+
+        let info;
+        if (
+          box_type != "BlockLevelBoxes" &&
+          box_type != "InlineFormattingContext"
+        ) {
+          info = Object.assign({}, Object.values(container)[0]);
+          delete info.children;
+          delete info.contents;
+          delete info.tag;
+        }
+
+        return {
+          text,
+          nodes,
+          info
+        };
+      }
+
+      function box_tree_from_bfc(bfc) {
+        const { contains_floats, contents } = bfc;
+        let block_container = Object.values(contents)[0];
+        return {
+          text: "BlockFormattingContext",
+          info: {
+            contains_floats
+          },
+          nodes: [box_tree_from_container(contents)]
+        };
+      }
+
+      function create_fragment_tree(root) {
+        let fragment = Object.values(root)[0];
+        let node = {
+          text: Object.keys(root)[0],
+          id: fragment.debug_id,
+          href: "#diff-" + fragment.debug_id
+        };
+
+        if (fragment.children) {
+          let children = [];
+          for (let i = 0; i < fragment.children.length; ++i) {
+            children.push(create_fragment_tree(fragment.children[i]));
+          }
+
+          if (children.length > 0) {
+            node.nodes = children;
+          }
+        }
+
+        node.info = Object.assign({}, fragment);
+        delete node.info.children;
+        delete node.info.debug_id;
+
+        return node;
+      }
+
+      function flatten_trace(trace_node) {
+        const fragment_tree_root = Object.values(
+          trace_node.fragment_tree.children
+        )[0];
+        return {
+          fragment_tree: create_fragment_tree(fragment_tree_root),
+          box_tree: box_tree_from_bfc(trace_node.box_tree)
+        };
+      }
+
+      function create_trace_tree_node(trace_node) {
+        const trace = flatten_trace(trace_node.pre);
+
+        let tree_node = {
+          text: trace_node.name,
+          icon: "dummy",
+          box_tree: trace.box_tree,
+          fragment_tree: trace.fragment_tree
+        };
+
+        let node = Object.values(trace_node)[0];
+        if (node.children) {
+          let children = [];
+          for (let i = 0; i < node.children.length; ++i) {
+            children.push(create_trace_tree_node(node.children[i]));
+          }
+
+          if (children.length > 0) {
+            tree_node.nodes = children;
+          }
+        }
+
+        return tree_node;
+      }
+
+      function new_data_loaded(data) {
+        jsondiffpatch.formatters.html.hideUnchanged();
+
+        let node_color_hash = {};
+        let tree = [create_trace_tree_node(data)];
+        $("#trace-tree").treeview({ data: tree, levels: 3 });
+        $("#trace-tree").on("nodeSelected", function(event, node) {
+          $("#fragment-diffs").empty();
+          $("#trace-tree")
+            .treeview(true)
+            .revealNode(node);
+
+          const on_tree_node_selected = tree => (event, data) => {
+            $(`#${tree}-diffs`).empty();
+            if (!data.info) return;
+            // XXX(ferjm) no diff for now.
+            const delta = jsondiffpatch
+              .create({
+                objectHash: function(obj) {
+                  return JSON.stringify(obj);
+                }
+              })
+              .diff({}, data.info);
+
+            const json = jsondiffpatch.formatters.html.format(delta, data.info);
+
+            $(`#${tree}-diffs`).append(
+              "<div class='panel panel-default'><div class='panel-heading'>" +
+                data.text +
+                "</div><div class='panel-body'>" +
+                json +
+                "</div></div>"
+            );
+          };
+
+          const on_fragment_tree_node_selected = on_tree_node_selected(
+            "fragment"
+          );
+          const on_box_tree_node_selected = on_tree_node_selected("box");
+
+          $("#fragment-tree").treeview({
+            data: [node.fragment_tree],
+            levels: 100,
+            enableLinks: false,
+            emptyIcon: "glyphicon glyphicon-unchecked hidden-glyphicon",
+            onNodeSelected: on_fragment_tree_node_selected
+          });
+
+          $("#box-tree").treeview({
+            data: [node.box_tree],
+            levels: 100,
+            enableLinks: false,
+            emptyIcon: "glyphicon glyphicon-unchecked hidden-glyphicon",
+            onNodeSelected: on_box_tree_node_selected
+          });
+
+          ["fragment", "box"].forEach(key => {
+            const collapsable = $(`#${key}-tree-collapse`);
+            collapsable.html("Collapse all").on("click", () => {
+              const collapsed = collapsable.data("collapsed");
+              if (collapsed == 0) {
+                $(`#${key}-tree`).treeview("collapseAll");
+              } else {
+                $(`#${key}-tree`).treeview("expandAll");
+              }
+              collapsable.html(collapsed == 0 ? "Expand all" : "Collapse all");
+              collapsable.data("collapsed", collapsed == 0 ? 1 : 0);
+            });
+          });
+        });
+
+        $("#trace-tree")
+          .treeview(true)
+          .selectNode(0);
+      }
+
+      $(document).ready(function() {
+        let upload = document.getElementsByTagName("input")[0];
+        upload.onchange = function(e) {
+          e.preventDefault();
+
+          let file = upload.files[0],
+            reader = new FileReader();
+          reader.onload = function(event) {
+            new_data_loaded(JSON.parse(event.target.result));
+          };
+
+          reader.readAsText(file);
+          return false;
+        };
+      });
+    </script>
+  </body>
+</html>
