Prevent duplicate signals from being generated for top-level LayerMod…

…el. (#5350)

When a LayerModel is at the top-level, top-level signals and unit model
signals all get flattened into the same array. As a result, we would
have bound projection signals alongside their "push: outer" siblings.
Although technically incorrect, this approach worked fine up until
Vega 5.5, when a change to the dataflow was introduced.

src/compile/selection/transforms/scales.ts

@@ -7,6 +7,7 @@ import {accessPathWithDatum, varName} from '../../../util';
 import {UnitModel} from '../../unit';
 import {SelectionProjection} from './project';
 import {TransformCompiler} from './transforms';
+import {isLayerModel, Model} from '../../model';
 
 const scaleBindings: TransformCompiler = {
   has: selCmpt => {
@@ -48,7 +49,7 @@ const scaleBindings: TransformCompiler = {
 
     // Top-level signals are only needed for multiview displays and if this
     // view's top-level signals haven't already been generated.
-    if (!model.parent || !bound.length) {
+    if (!model.parent || isTopLevelLayer(model) || !bound.length) {
       return signals;
     }
 
@@ -77,7 +78,7 @@ const scaleBindings: TransformCompiler = {
 
   signals: (model, selCmpt, signals) => {
     // Nested signals need only push to top-level signals with multiview displays.
-    if (model.parent) {
+    if (model.parent && !isTopLevelLayer(model)) {
       for (const proj of selCmpt.scales) {
         const signal: any = signals.filter(s => s.name === proj.signals.data)[0];
         signal.push = 'outer';
@@ -96,3 +97,7 @@ export function domain(model: UnitModel, channel: Channel) {
   const scale = stringValue(model.scaleName(channel));
   return `domain(${scale})`;
 }
+
+function isTopLevelLayer(model: Model): boolean {
+  return model.parent && isLayerModel(model.parent) && (!model.parent.parent || isTopLevelLayer(model.parent.parent));
+}