This reduces the jumpiness on insertion and deletion. For nested sequences, it's not possible to eliminate the jump entirely without editing the CSS, because the insert button is `display: inline-block` which gives it a minimum height of the containing block's line-height - as a result, this spacing appears immediately as soon as the block changes from display:none to visible.
While focusing on the first child field of a new block is generally a good thing, focusing a date field opens the date picker, which looks clunky when you've not explicitly requested it. Introduce a 'soft' focus flag to indicate that we only want to focus the field if it can be done unobtrusively.
This will allow packages such as Wagtail to define their own 'sandboxed' registries containing package-specific adapters - in Wagtail's case we want this so that we can take over the details of serving static JS (so that we don't need to add telepath to INSTALLED_APPS and can use the Wagtail-specific versioned_static helper).
This method will be suitable for pulling out into a common superclass. Having matching method signatures for ListBlock._insert and StreamBlock_insert means that the glue code for handling onRequestInsert can be shared, while the public insert methods of ListBlock and StreamBlock remain distinct (because having to pass a block type / definition to ListBlock.insert would be a pretty bad public API).
This starts to make the insert logic generic, and irons out the awkwardness of having to pass groupedChildBlockDefs as an extra option on StreamBlockMenu
This makes it generic to both StreamBlock and ListBlock: the callback passes an index number and a dict of container-specific options (empty for ListBlock, and a 'type' option for StreamBlock)
The "initial menu for an empty stream is initially open" business rule is implemented elsewhere, in StreamBlock.setState, so this is redundant in the constructor - and it will get in the way of sharing implementation with ListBlock, because the equivalent control for ListBlock (i.e. the Add button) doesn't have a concept of open-ness.