Notices

You should be able to release your finger after the menu pops up. Unfortunately in the browser it tends to close the menu again...

Many right handed people hold their phones in their left hand by the way, not that this helps much with this anyway. In general, it's best if popups are avoided in the UI.

Even having the menu appear such that one's finger is at the bottom-left corner*, rather than top-left, might be an improvement. Yes, I'm sure designers have thoughtfully optimized the ordering of menu items from top to bottom, most to least used. But like the cat in the box, does a menu hidden behind a thumb have any ordering for its items?

* Space permitting, blah, blah.