Forward slash inserter doesn't work in the widgets editor

[draganescu]

When using the forward slash inserter in the new Widgets editor nothing happens.

To reproduce

1. Use the latest WordPress 5.9 RC
2. Go to Appearance > Widgets
3. Add a paragraph block
4. Type "/"
5. Observe nothing happens

Screenshots

slash-inserter.mp4

Expected behavior

The slash inserter should behave like the one in the post editor, triggering the quick insert popover menu.

[tellthemachines]

I can only reproduce this when starting from an empty widget area and typing consecutive paragraphs. Interestingly enough, as soon as I hover over the widget area, causing the in-between inserter to appear, the autocomplete box shows up.

[tellthemachines]

So this is happening because at the point the popover menu tries to render, the list of valid blocks for insertion is empty, because templateLock is set to all (the filter is returning false here). I don't know why that's the case though, as we're setting templateLock to false in the widget area inner block props.

To make things worse, it doesn't happen every single time. It only happens when adding blocks to an empty widget area, but maybe one time out of ten it works as expected 😖