Keep topology nav visible if selected

If the scope-app API unexpectedly restarts, it has no report
at hand (until it gets one from the probe) and sends node
count 0 to the frontend for all topologies. Once the report
arrives, it will send the proper count.

What happened was the frontend did hide Processes for a short
time till the node count recovered. This moved the topology
selection to the always visible Containers (hide_if_empty == false)
while keeping the graph as is.

Once the node count recovers, Processes comes back but the
selection is still at Containers.

We now keep the selected topology visible at all time even if
the API returns a node count of 0. This recovers nicely when
the correct node counts come in. Once the user selects a different
topology while and a backend response arrives, it disappears.

Fixes #2646

client/app/scripts/reducers/__tests__/root-test.js

@@ -280,6 +280,25 @@ describe('RootReducer', () => {
     }]
   };
 
+  const ReceiveTopologiesHiddenAction = {
+    type: ActionTypes.RECEIVE_TOPOLOGIES,
+    topologies: [{
+      url: '/topo1',
+      name: 'Topo1',
+      stats: {
+        node_count: 1
+      }
+    }, {
+      hide_if_empty: true,
+      url: '/topo2',
+      name: 'Topo2',
+      stats: {
+        node_count: 0,
+        filtered_nodes: 0
+      }
+    }]
+  };
+
   const RouteAction = {
     type: ActionTypes.ROUTE_TOPOLOGY,
     state: {}
@@ -567,6 +586,21 @@ describe('RootReducer', () => {
     expect(isTopologyNodeCountZero(nextState)).toBeFalsy();
   });
 
+  it('keeps hidden topology visible if selected', () => {
+    let nextState = initialState;
+    nextState = reducer(nextState, ClickTopology2Action);
+    nextState = reducer(nextState, ReceiveTopologiesHiddenAction);
+    expect(nextState.get('currentTopologyId')).toEqual('topo2');
+    expect(nextState.get('topologies').toJS().length).toEqual(2);
+  });
+
+  it('keeps hidden topology hidden if not selected', () => {
+    let nextState = initialState;
+    nextState = reducer(nextState, ClickTopologyAction);
+    nextState = reducer(nextState, ReceiveTopologiesHiddenAction);
+    expect(nextState.get('topologies').toJS().length).toEqual(1);
+  });
+
   // selection of relatives
 
   it('keeps relatives as a stack', () => {

client/app/scripts/reducers/root.js

@@ -110,17 +110,17 @@ function calcSelectType(topology) {
 // adds ID field to topology (based on last part of URL path) and save urls in
 // map for easy lookup
 function processTopologies(state, nextTopologies) {
+  // add IDs to topology objects in-place
+  const topologiesWithId = updateTopologyIds(nextTopologies);
   // filter out hidden topos
-  const visibleTopologies = filterHiddenTopologies(nextTopologies);
+  const visibleTopologies = filterHiddenTopologies(topologiesWithId, state.get('currentTopologyId'));
   // set `selectType` field for topology and sub_topologies options (recursive).
   const topologiesWithSelectType = visibleTopologies.map(calcSelectType);
-  // add IDs to topology objects in-place
-  const topologiesWithId = updateTopologyIds(topologiesWithSelectType);
   // cache URLs by ID
   state = state.set('topologyUrlsById',
-    setTopologyUrlsById(state.get('topologyUrlsById'), topologiesWithId));
+    setTopologyUrlsById(state.get('topologyUrlsById'), topologiesWithSelectType));
 
-  const topologiesWithFullnames = addTopologyFullname(topologiesWithId);
+  const topologiesWithFullnames = addTopologyFullname(topologiesWithSelectType);
   const immNextTopologies = fromJS(topologiesWithFullnames).sortBy(topologySorter);
   return state.set('topologies', immNextTopologies);
 }

client/app/scripts/utils/topology-utils.js

@@ -126,9 +126,9 @@ export function setTopologyUrlsById(topologyUrlsById, topologies) {
   return urlMap;
 }
 
-export function filterHiddenTopologies(topologies) {
+export function filterHiddenTopologies(topologies, currentTopologyId) {
   return topologies.filter(t => (!t.hide_if_empty || t.stats.node_count > 0 ||
-                               t.stats.filtered_nodes > 0));
+                               t.stats.filtered_nodes > 0 || t.id === currentTopologyId));
 }
 
 export function getCurrentTopologyOptions(state) {