Allow Unit rebase to top-level

Outline base Unit row during drag
Closes #25

app/views/units/index.html.erb

@@ -18,6 +18,12 @@
         <th></th>
       <% end %>
     </tr>
+    <%= tag.tr id: 'unit_', hidden: true,
+               ondragover: 'dragOver(event)', ondrop: 'drop(event)',
+               ondragenter: 'dragEnter(event)', ondragleave: 'dragLeave(event)',
+               data: {drop_id: 'unit_'} do %>
+      <th colspan="5"><%= t '.top_level_drop' %></th>
+    <% end %>
   </thead>
   <tbody id="units">
     <%= render(@units) || render_no_items %>
@@ -31,17 +37,54 @@
     }
   }
 
+  var lastEnterTime;
   function dragStart(event) {
-    var row = event.target.closest("tr");
+    lastEnterTime = event.timeStamp;
+    var row = event.currentTarget;
+    row.closest("table").querySelectorAll("thead tr").forEach((tr) => {
+      tr.toggleAttribute("hidden");
+    });
     event.dataTransfer.setData("text/plain", row.getAttribute("data-drag-path"));
     var rowRectangle = row.getBoundingClientRect();
     event.dataTransfer.setDragImage(row, event.x - rowRectangle.left, event.y - rowRectangle.top);
-    event.dataTransfer.dropEffect = "none";
+    event.dataTransfer.dropEffect = "move";
+  }
+
+  /*
+  * Drag tracking assumptions (based on FF 122.0 experience):
+  *   * Enter/Leave events at the same timeStamp may not be logically ordered
+  *     (e.g. E -> E -> L, not E -> L -> E),
+  *   * not every Enter event has corresponding Leave event, especially during
+  *     rapid pointer moves
+  * NOTE: sometimes Leave is not emitted when pointer goes fast over table
+  * and outside. This should probably be fixed in browser, than patched here.
+  */
+  function dragEnter(event) {
+    //console.log(event.timeStamp + " " + event.type + ": " + event.currentTarget.id);
+    dragLeave(event);
+    lastEnterTime = event.timeStamp;
+    const id = event.currentTarget.getAttribute("data-drop-id");
+    document.getElementById(id).classList.add("dropzone");
   }
 
   function dragOver(event) {
     event.preventDefault();
-    event.dataTransfer.dropEffect = "move";
+  }
+
+  function dragLeave(event) {
+    //console.log(event.timeStamp + " " + event.type + ": " + event.currentTarget.id);
+    // Leave has been accounted for by Enter at the same timestamp, processed earlier
+    if (event.timeStamp <= lastEnterTime) return;
+    event.currentTarget.closest("table").querySelectorAll(".dropzone").forEach((tr) => {
+      tr.classList.remove("dropzone");
+    })
+  }
+
+  function dragEnd(event) {
+    dragLeave(event);
+    event.currentTarget.closest("table").querySelectorAll("thead tr").forEach((tr) => {
+      tr.toggleAttribute("hidden");
+    });
   }
 
   function drop(event) {