Introduce range finder

Summary: Show a hint during scrolling of the relative offset the user is looking at. This is based on the current virtualisation window, so not 100% accurate, but probably still provides the right signal to the user. See the bottom right of the recording

Reviewed By: nikoant

Differential Revision: D26450261

fbshipit-source-id: 206a860024e346c6b872edc3fc7919019046a6d7

desktop/flipper-plugin/src/state/datasource/DataSource.tsx

@@ -91,8 +91,8 @@ export class DataSource<
 
   private dataUpdateQueue: DataEvent<T>[] = [];
 
-  private windowStart = 0;
-  private windowEnd = 0;
+  windowStart = 0;
+  windowEnd = 0;
 
   private outputChangeListener?: (change: OutputChange) => void;