Remove margins for spacer elements in thread list

These elements are used purely to reserve space for offscreen threads
which are not actually present in the DOM, so they should not occupy any
space beyond that assigned to their 'height' inline style properties.

h/static/styles/thread.scss

 $thread-padding: $annotation-card-left-padding;
 
-.stream-list {
+.thread-list {
   & > * {
     margin-bottom: .72em;
   }
@@ -10,6 +10,13 @@ $thread-padding: $annotation-card-left-padding;
   }
 }
 
+.thread-list__spacer {
+  // This is a hidden element which is used to reserve space for off-screen
+  // threads, so it should not occupy any space other than that set via its
+  // 'height' inline style property.
+  margin: 0;
+}
+
 .annotation-unavailable-message {
   display: flex;
   flex-direction: column;

h/templates/client/viewer.html

@@ -2,7 +2,7 @@
 
 (See gh2642 for rationale for 'ng-show="true"')
  -->
-<ul class="stream-list ng-hide"
+<ul class="thread-list ng-hide"
     ng-show="true"
     window-scroll="loadMore(20)">
   <search-status-bar
@@ -30,7 +30,8 @@
       You do not have permission to see this annotation
     </p>
   </li>
-  <li ng-style="{height: virtualThreadList.offscreenUpperHeight}"></li>
+  <li class="thread-list__spacer"
+      ng-style="{height: virtualThreadList.offscreenUpperHeight}"></li>
   <li id="{{child.id}}"
       class="annotation-card thread"
       ng-class="{'js-hover': hasFocus(child.annotation)}"
@@ -45,7 +46,8 @@
         on-force-visible="forceVisible(thread)">
       </annotation-thread>
   </li>
-  <li ng-style="{height: virtualThreadList.offscreenLowerHeight}"></li>
+  <li class="thread-list__spacer"
+      ng-style="{height: virtualThreadList.offscreenLowerHeight}"></li>
   <loggedout-message ng-if="isSidebar && shouldShowLoggedOutMessage()"
     on-login="login()" ng-cloak>
   </loggedout-message>