The canvas with a run around it: statifier-ui's status and scrubber above it, its event log below, and the document's own blocks where an ops view would draw a diagram.

An ops view built out of StatifierUI.Live draws four things - a status header, a scrubber, a Mermaid diagram of the chart, and the event log. Three of those are about the run and are the same wherever the run is watched from. The fourth is about the chart, and inside this editor there is something better to put in its seat: the blocks the author wrote, already laid out, already marked. So the diagram is not mounted, the canvas takes that seat, and the three run surfaces are composed around it exactly as statifier-ui ships them.

The seam is a read, and it stays one

statifier-ui owns the trace wire format and the components that draw it; this package owns blocks, the compiler and its provenance map. Nothing in that split needed a callback: the components take a StatifierUI.Live.State and draw it, StatifierBlocks.Runtime.Marks turns the same read model into the marks the canvas already accepts, and StatifierBlocks.Runtime.Handled turns a clicked log entry into the block whose state handled it. No statifier-ui module changed, no new wire type exists, and the two events the components emit are statifier-ui's own - renamed by the caller, which is what scrub_event and select_event are for.

Optional, and quiet when absent

statifier_ui is an optional dependency here, exactly as phoenix_live_view is. The three components are resolved at runtime from a module read out of :statifier_blocks, :run_pane_module and checked with function_exported?/3 - the indirection StatifierBlocks.Editor.Field uses for the expression input and StatifierBlocks.Runtime.Marks uses for the inspector reads, and for the same two reasons: the compiler stays quiet in a tree without the package, and a test can point the key at a module of its own to exercise the absent branch on a machine where the package is present.

With nothing resolvable the pane still draws, still seats the canvas, and says in one line that the run surfaces need the package. It does not refuse to render: an author whose host forgot a dependency should still see their document.

A pane with no run is not a pane

run_pane/1 with state: nil renders its inner block and nothing else - no wrapper element, no class, no attribute. A document with no run over it therefore produces exactly the markup it produced before this component existed, which is a property the editor's tests assert rather than assume.