fix: Improve flyout behavior

[gonfunko]

The basics

• I validated my changes

The details

Resolves

• Fixes Remove explicit variable handling from flyout #8547
• Fixes Dragging in from the flyout sometimes bumps unexpectedly #8769
• Fixes It's harder to drag out blocks if the flyout has a scrollbar #9239

Proposed Changes

This PR is a bit of a grab bag of flyout related fixes. Specifically, it:

• Removes a bunch of special handling of variable and block creation events when dragging a block out of the flyout. Experimentally this still seems to work fine, including undo, and including blocks that reference potential variables, and all tests still pass.
• Avoids a race condition that could result in inadvertent block bumps when dragging out a new block by calling appendInternal() instead of append(), the latter of which triggers an immediate (and unnecessary) render before the state of things is fully combobulated.
• Changes the behavior such that dragging on a block will not ever drag-to-scroll the flyout. The flyout can still be scrolled by clicking and dragging on the flyout workspace background, but clicks targeting blocks will always be treated as a drag on the block.

[maribethb]

If the block is not successfully created then now the flyout is closed, where previously the event utils would be enabled but then the method would crash (without closing flyout)

I think if a block isn't ultimately created for whatever reason (not sure how likely this method is to throw an error in reality) it would be better for the flyout not to close.

i don't think you need to use a try anymore since we don't have to clean up the event utils. if placeNewBlock errors then just let it error. so you can hideChaff before you return the new block without relying on the finally block.

[gonfunko]

Good catch, updated; this left this method as nothing but hideChaff() and calling through to placeNewBlock(), so I inlined placeNewBlock() here as it was private and had no other callsites.

[maribethb]

Doesn't hiding the chaff on the target workspace already close the flyout?

[gonfunko]

Indeed it does, this was left over from before, but removed accordingly!

[maribethb]

otherwise lgtm