[Cashtab] Patch minify menu flicker

Summary:
T3508

Still seeing some break points where the menu will flicker between minified and unminified, based on scrollY changing to and from opposite sides of the threshhold during browser-calculated re-renders.

Update the logic so that we do not switch back to the "full" menu until we scroll further up.

Test Plan: npm test, npm start try to recreate the issue

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15981