How to use State.tryFindByKey with index in Avalonia.FuncUI (components)?

General
1

Context:

let dataRows: IWritable<DataRow list> = ctx.usePassed dataRowsSrc
let selectedIdx: IWritable<ing> = ctx.useState 0

I found this approach from an example to “derive” an IWritable

let selectedDataRow =
	dataRows
	|> State.tryFindByKey (fun t -> t.someKey) selectedIdx // works perfectly fine if i replace someKey with something real and selectedIdx with a matching IWritable 
	|> ctx.usePassed

if i try to get the index like in this example.

...
|> State.tryFindByKey (fun (idx, _) -> idx) // this results in IWritable<(DataRow list * obj) option>
...

What am I missing? Please correct me if some part of this approach is misguided.

Larger application context:

Main:

let dataRowsSrc = new State<_>(data |> List.sortBy (_.name))
...
let dataRows: IWritable<DataRow list> = ctx.usePassed dataRowsSrc
let selectedIdx: IWritable<int> = ctx.useState 0

let selectedDataRow =
	dataRows
	|> State.tryFindByKey (fun (idx, _) -> idx) selectedIdx // KABOOM!
	|> ctx.usePassed
...
StackPanel.create [
	StackPanel.orientation Orientation.Horizontal
	StackPanel.children [
		dataRowsListView (dataRows, selectedIdx)
		dataRowDetailsView selectedDataRow
	]
]
...

ListView:

...
let dataRowsListView (rows: IReadable<DataRow list>, selectedIdx: IWritable<int>) =
Component.create ("dataRow list", fun ctx ->
	 let rows = ctx.usePassedRead rows
	 let selectedIdx = ctx.usePassed selectedIdx
	 ...
	 DockPanel.create [
		...
		DockPanel.children [
			ListBox.create [
				ListBox.dock Dock.Top
				ListBox.dataItems rows.Current
				ListBox.itemTemplate (
					DataTemplateView.create<_, _>(fun data ->
						TextBlock.create [
							TextBlock.text data.name
						]
					)
				)
				ListBox.onSelectedIndexChanged selectedIdx.Set
			]
		]
	]
)
...

DetailsView:

let dataRowDetailsView (dataRow: IWritable<DataRow option>) =
	Component.create ($"dataRow details", fun ctx ->
		let data = ctx.usePassed dataRow
...

Many thanks!