I am seeing the following React warning in the Browser Console when stepping with Inline previews enabled.

Warning: Each child in a list should have a unique "key" prop.

Check the render method of `InlinePreviews`. See https://fb.me/react-warning-keys for more information.
    in Connect(InlinePreviewRow) (created by InlinePreviews)
    in InlinePreviews (created by Connect(InlinePreviews))
    in Connect(InlinePreviews) (created by Editor)
    in div (created by Editor)
    in div (created by Editor)
    in Editor (created by Connect(Editor))
    in Connect(Editor) (created by App)
    in div (created by App)
    in div (created by App)
    in div (created by SplitBox)
    in div (created by SplitBox)
    in SplitBox (created by App)
    in div (created by SplitBox)
    in div (created by SplitBox)
    in SplitBox (created by App)
    in div (created by A11yIntention)
    in A11yIntention (created by App)
    in div (created by App)
    in App (created by Connect(App))
    in Connect(App)
    in Provider
    in Provider

Honza

https://hg.mozilla.org/mozilla-central/rev/d0133dad2708