Prevent accidental inheritance of previous context on undefined context

Author: jacbn

src/app/services/pageContext.ts

@@ -163,6 +163,7 @@ export function useUrlPageTheme(): PageContextState {
         dispatch(pageContextSlice.actions.updatePageContext({
             subject: urlContext?.subject, 
             stage: urlContext?.stage,
+            previousContext: {subject: urlContext?.subject, stage: urlContext?.stage},
         }));
 
         return () => {

src/app/state/slices/context.ts

@@ -12,8 +12,9 @@ export const pageContextSlice = createSlice({
     reducers: {
         updatePageContext: (state, action: actionType) => ({
             ...state,
-            stage: action.payload?.stage ?? state?.stage,
-            subject: action.payload?.subject ?? state?.subject,
+            // stage and subject can be undefined, so should not ??-inherit from the previous context
+            stage: action.payload?.stage,
+            subject: action.payload?.subject,
             previousContext: action.payload?.previousContext ?? state?.previousContext,
         }),
         resetPageContext: (state) => ({