Add some Partial<> annotations.

Without these, it thinks that all of the possible keys are present.

Fixing this is needed if we want to avoid '@typescript-eslint/no-unnecessary-condition'
complaining about our 'prop in obj' tests.

Author: mstange

src/reducers/app.ts

@@ -176,7 +176,7 @@ const lastVisibleThreadTabSlug: Reducer<TabSlug> = (
   }
 };
 
-const trackThreadHeights: Reducer<{ [key: ThreadsKey]: CssPixels }> = (
+const trackThreadHeights: Reducer<Partial<Record<ThreadsKey, CssPixels>>> = (
   state = {},
   action
 ) => {

src/selectors/app.tsx

@@ -58,9 +58,9 @@ export const getPanelLayoutGeneration: Selector<number> = (state) =>
   getApp(state).panelLayoutGeneration;
 export const getLastVisibleThreadTabSlug: Selector<TabSlug> = (state) =>
   getApp(state).lastVisibleThreadTabSlug;
-export const getTrackThreadHeights: Selector<{
-  [key: ThreadsKey]: CssPixels;
-}> = (state) => getApp(state).trackThreadHeights;
+export const getTrackThreadHeights: Selector<
+  Partial<Record<ThreadsKey, CssPixels>>
+> = (state) => getApp(state).trackThreadHeights;
 export const getIsNewlyPublished: Selector<boolean> = (state) =>
   getApp(state).isNewlyPublished;
 export const getExperimental: Selector<ExperimentalFlags> = (state) =>

src/types/state.ts

@@ -180,7 +180,7 @@ export type AppState = {
   readonly sidebarOpenCategories: Map<string, Set<number>>;
   readonly panelLayoutGeneration: number;
   readonly lastVisibleThreadTabSlug: TabSlug;
-  readonly trackThreadHeights: Record<ThreadsKey, CssPixels>;
+  readonly trackThreadHeights: Partial<Record<ThreadsKey, CssPixels>>;
   readonly isNewlyPublished: boolean;
   readonly isDragAndDropDragging: boolean;
   readonly isDragAndDropOverlayRegistered: boolean;

src/types/transforms.ts

@@ -350,4 +350,6 @@ export type Transform = TransformDefinitions[keyof TransformDefinitions];
 export type TransformType = Transform['type'];
 
 export type TransformStack = Transform[];
-export type TransformStacksPerThread = { [key: ThreadsKey]: TransformStack };
+export type TransformStacksPerThread = Partial<
+  Record<ThreadsKey, TransformStack>
+>;