[hufwin]

Fixes the crash discussed on Discord (keyed node in a child list whose
shape changes across an update).

_VirtualDom_dekey still read keyedKids[i].b — the field of Elm's
(key, node) tuples. Gren's keyed children are { key, node } records,
which every other kernel site reads through __$node (_VirtualDom_render,
the keyed constructor's descendants count). On the diff path where an old
plain node meets a new keyed node at the same index, dekey therefore
produced a node whose children were all undefined; the following redraw
crashed with Cannot read properties of undefined (reading '$'), and from
then on every render threw removeChild type errors as the real DOM and
the virtual one drifted apart.

Minimal reproduction (click once):

view : Bool -> Html {}
view toggled =
    Html.div []
        ((if toggled then
            [ Html.div [] [ Html.text "moved" ]
            , Html.div []
                [ Html.button [ Html.Events.onClick {} ] [ Html.text "toggle" ] ]
            ]

          else
            [ Html.button [ Html.Events.onClick {} ] [ Html.text "toggle" ]
            , Html.text "flat"
            , Html.div [] [ Html.text "filler" ]
            ]
         )
            ++ [ Html.Keyed.node "span"
                    []
                    [ { key = "only", node = Html.text "keyed" } ]
               ]
        )

Verified by patching a package-cache copy of the kernel: the untouched
6.0.2 crashes on the first update, this change alone stops it, and
reverting brings the crash back. gren make compiles all 24 modules with
the change.

[robinheghan]

The fix looks good, but I haven't had the time to run the minimal repro case to verify. Will try to make time in the coming days.

[gilramir]

For what it's worth, I ran into the same problem last night in a little web app I'm writing.

My HTML body has a Just/Nothing when expression:

Html.main_ [ Attr.id "doc-pane" ]
    [ when model.current is
        Just doc ->
            viewNode doc.root      -- root case renders Html.Keyed.node "article" ...

        Nothing ->
            Html.p [ Attr.class "empty" ] [ Html.text "Select a document." ]
    ]

The first render (the Nothing case) was fine. When I clicked the page and model.current became Just, the page
died with:

TypeError: Cannot read properties of undefined (reading '$')
    at _VirtualDom_render
    at _VirtualDom_render
    at _VirtualDom_applyPatchRedraw
    at _VirtualDom_applyPatch
    at _VirtualDom_applyPatchesHelp

Instrumenting the compiled output at the point of the
crash shows a dekeyed parent whose children are all undefined:

BAD PARENT: $=1 typeof=number c=article nodeKeys=["$","c","d","e","f","b"] kids=["UNDEF"]

$=1 with c=article is the dekeyed node.

I patched my compiled javascript with the keyedKids[i].b -> keyedKids[i].node fix and it works.