[PM] Leave nodes pinned after dragging

They can be un-pinned by dropping them outside their designated area.

R=siggi

Change-Id: I719d6be9509666f4387a4bba91946c3861b771eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2214913Reviewed-by: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Auto-Submit: Joe Mason <joenotcharles@chromium.org>
Commit-Queue: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771910}

chrome/browser/resources/discards/graph_doc.js

@@ -993,8 +993,13 @@ class Graph {
     if (!d3.event.active) {
       this.simulation_.alphaTarget(0);
     }
-    d.fx = null;
-    d.fy = null;
+    // Leave the node pinned where it was dropped. Return it to free
+    // positioning if it's dropped outside its designated area.
+    const bounds = d.allowedYRange(this.height_);
+    if (d3.event.y < bounds[0] || d3.event.y > bounds[1]) {
+      d.fx = null;
+      d.fy = null;
+    }
   }
 
   /**