fix: Fix scrolling of menus on mobile.

[gonfunko]

The basics

• I branched from develop
• My pull request is against develop
• My code follows the style guide
• I ran npm run format and npm run lint

The details

Resolves

Fixes #6759

Proposed Changes

The menu item selection handler is triggered on pointerup vs down.

Behavior Before Change

Attempting to scroll a menu on mobile would select the item that was tapped to initiate the scroll.

Behavior After Change

Menus can be scrolled as expected, and tapping an item selects it.

[maribethb]

Just checking for my own understanding-
when you scroll, it doesn't select the item you're hovering over when you stop scrolling and lift your finger, right? and that works because the browser is smart enough to issue a pointercancel event once the element is scrolled? I couldn't find Blockly-specific code that's cancelling it but after a quick review of the pointer events spec that's what I think is happening, and wanted to confirm that's your understanding as well.

[gonfunko]

Correct, experimentally it behaves as you describe/would expect - the item you let up on isn't selected, you have to tap (separate from initiating/ending the scroll) to select one.