Fix height cache (#4909)

app/javascript/mastodon/reducers/height_cache.js

@@ -0,0 +1,23 @@
+import { Map as ImmutableMap } from 'immutable';
+import { HEIGHT_CACHE_SET, HEIGHT_CACHE_CLEAR } from '../actions/height_cache';
+
+const initialState = ImmutableMap();
+
+const setHeight = (state, key, id, height) => {
+  return state.update(key, ImmutableMap(), map => map.set(id, height));
+};
+
+const clearHeights = () => {
+  return ImmutableMap();
+};
+
+export default function statuses(state = initialState, action) {
+  switch(action.type) {
+  case HEIGHT_CACHE_SET:
+    return setHeight(state, action.key, action.id, action.height);
+  case HEIGHT_CACHE_CLEAR:
+    return clearHeights();
+  default:
+    return state;
+  }
+};