[form-builder] Fix portable text editor scroll bug where the editor s…

…crolled to top after acquiring focus. (#1562)

packages/@sanity/form-builder/src/inputs/BlockEditor/plugins/OnFocusPlugin.ts

@@ -1,13 +1,22 @@
 // This plugin runs when the editor receives focus
 // Ensures that an empty block is inserted when the editor gets focus and it's empty.
 
+// It also prevents default onFocus actions in slate-react being triggered as we
+// abort the plugin chain. The version of slate-react we are currently using (0.21.16)
+// has a terrible scrolling bug which will be prevented by canceling the plugin chain here.
+
+// TODO: remove this and continue once we have upgraded slate-react
+// (where the bug is fixed - see https://github.com/ianstormtaylor/slate/commit/d28f78c0607ad147f85ff4a363fa65d7ddfa7c8c)
+
 import {SlateEditor} from '../typeDefs'
 
 export default function OnFocusPlugin() {
   return {
     onFocus(event: any, editor: SlateEditor, next: (arg0: void) => void) {
       editor.command('ensurePlaceHolderBlock')
-      return next()
+      editor.focus()
+      // Cancel slate-react's 'after' plugin chain to prevent built in scrolling bug.
+      return true
     }
   }
 }