[entropy] don't update if not on screen

Use an intersection observer to detect when the entropy panel is visible
on the screen (viewport). When it's offscreen we don't update entropy
data (an expensive calculation). When it comes back onscreen we
recalculate the entropy data if it has become stale.

This results in slightly strange behaviour when the entropy panel will
be shown with no bars and they'll be drawn after a slight delay (while
the data's recalculated). The wins are much improved performance when
the entropy panel is not on screen (which is common).

src/actions/entropy.js

@@ -14,7 +14,7 @@ export const updateEntropyVisibility = debounce((dispatch, getState) => {
     controls.animationPlayPauseButton !== "Play"
   ) {return;}
 
-  if (!controls.panelsToDisplay.includes("entropy")) {
+  if (!controls.panelsToDisplay.includes("entropy") || entropy.onScreen===false) {
     if (entropy.bars===undefined) {
       return; // no need to dispatch another action - the state's already been invalidated
     }

src/actions/recomputeReduxState.js

@@ -1014,6 +1014,7 @@ export const createStateFromQueryOrJSONs = ({
     // The <Entropy> panel will calculate the bars/maxYVal if needed, so we defer calculation until then
     entropy.bars = undefined; 
     entropy.maxYVal = 1;
+    entropy.onScreen = true;
   }
 
   /* update frequencies if they exist (not done for new JSONs) */

src/actions/types.js

@@ -35,6 +35,7 @@ export const CHANGE_TREE_ROOT_IDX = "CHANGE_TREE_ROOT_IDX";
 export const TOGGLE_NARRATIVE = "TOGGLE_NARRATIVE";
 export const ENTROPY_DATA = "ENTROPY_DATA";
 export const ENTROPY_COUNTS_TOGGLE = "ENTROPY_COUNTS_TOGGLE";
+export const ENTROPY_ONSCREEN_CHANGE = "ENTROPY_ONSCREEN_CHANGE";
 export const PAGE_CHANGE = "PAGE_CHANGE";
 export const MIDDLEWARE_ONLY_ANIMATION_STARTED = "MIDDLEWARE_ONLY_ANIMATION_STARTED";
 export const URL_QUERY_CHANGE_WITH_COMPUTED_STATE = "URL_QUERY_CHANGE_WITH_COMPUTED_STATE";

src/components/entropy/entropyD3.js

@@ -468,8 +468,12 @@ EntropyChart.prototype._highlightSelectedBars = function _highlightSelectedBars(
 /* draw the bars (for each base / aa) */
 EntropyChart.prototype._drawBars = function _drawBars() {
   if (!this.okToDrawBars) {return;}
+
   this._groups.mainBars.selectAll("*").remove();
 
+  // bars may be undefined (indicating the underlying data became stale)
+  if (!this.bars) {return;}
+
   /* Calculate bar width */
   const validXPos = this.scales.xMain.domain()[0]; // any value inside the scale's domain will do
   let barWidth = this.scales.xMain(validXPos+1) - this.scales.xMain(validXPos); // pixels between 2 nucleotides

src/components/entropy/index.js

@@ -12,7 +12,7 @@ import { tabGroup, tabGroupMember, tabGroupMemberSelected } from "../../globalSt
 import EntropyChart from "./entropyD3";
 import InfoPanel from "./infoPanel";
 import { changeEntropyCdsSelection, showCountsNotEntropy } from "../../actions/entropy";
-import { ENTROPY_DATA } from "../../actions/types";
+import { ENTROPY_DATA, ENTROPY_ONSCREEN_CHANGE } from "../../actions/types";
 import { timerStart, timerEnd } from "../../util/perf";
 import { encodeColorByGenotype } from "../../util/getGenotype";
 import { nucleotide_gene } from "../../util/globals";
@@ -70,6 +70,7 @@ const getStyles = (width) => {
     maxYVal: state.entropy.maxYVal,
     showCounts: state.entropy.showCounts,
     loaded: state.entropy.loaded,
+    onScreen: state.entropy.onScreen,
     colorBy: state.controls.colorBy,
     /**
      * Note that zoomMin & zoomMax only represent the state when changed by a URL
@@ -219,9 +220,28 @@ class Entropy extends React.Component {
     }
 
   }
+  visibilityOnScreenChange(entries) {
+    if (entries.length!==1) {
+      return console.error(`Unexpected IntersectionObserver callback entries of length`, entries.length);
+    }
+    const onScreen = entries[0].isIntersecting;
+    if (onScreen===this.props.onScreen) return; // can happen when component initially rendered
+    // if gone off screen or come back on screen with the bars still valid then it's a simple toggle action
+    if (!onScreen || this.props.bars) {
+      return this.props.dispatch({type: ENTROPY_ONSCREEN_CHANGE, onScreen})
+    }
+    // else if back on screen and the bars are invalid then we want to regenerate them
+    return this.props.dispatch((dispatch, getState) => {
+      const { entropy, tree } = getState();
+      const [bars, maxYVal] = calcEntropyInView(tree.nodes, tree.visibility, entropy.selectedCds, entropy.showCounts);
+      dispatch({type: ENTROPY_DATA, data: bars, maxYVal, onScreen});
+    });
+  }
   componentDidMount() {
     if (this.props.loaded) {
-      this.setUp(this.props);
+      this.setUp(this.props); 
+      const observer = new IntersectionObserver(this.visibilityOnScreenChange.bind(this), {threshold: 0.0});
+      observer.observe(this.d3entropy)
     }
   }
   UNSAFE_componentWillReceiveProps(nextProps) {

src/reducers/entropy.js

@@ -20,8 +20,11 @@ const Entropy = (state = defaultEntropyState(), action) => {
       return Object.assign({}, state, {
         loaded: true,
         bars: action.data,
-        maxYVal: action.maxYVal
+        maxYVal: action.maxYVal,
+        onScreen: Object.hasOwn(action, 'onScreen') ? action.onScreen : state.onScreen,
       });
+    case types.ENTROPY_ONSCREEN_CHANGE:
+      return {...state, onScreen: action.onScreen};
     case types.ENTROPY_COUNTS_TOGGLE:
       return Object.assign({}, state, {
         showCounts: action.showCounts