Debug DSH Web Plugin Runtime Behavior

External Web plugins call host client APIs whose contracts live in the DSH source tree, not in the plugin's own types. When behavior diverges from intent at runtime, the failure is almost always a misread contract — and the diagnosis must come from the host source, never from the API's name.

The standing rule: read the verb's contract in the host source first

Before changing any call into a host API, open the implementing package in the DSH source checkout (~/.dsh/source/current, or the vendored copy) and read the actual method — its doc comment, its guards, and the types it compares against. Repeat for every value the plugin passes. Three questions cover most incidents:

1. Which text does an offset count into? When a verb takes a span or an offset, find out what string those numbers index. Published snapshot fields and internal editor projections are not always the same string; a plugin that feeds one representation's offsets into a verb whose guard compares against another representation fails silently — the call returns false or no-ops, nothing throws.
2. What does one "unit" weigh in each representation? If the document contains opaque inline units (chips, tokens, attachments), check whether a unit occupies the same width in the published field as in the projection the verb guards. When widths differ, offsets are only correct while no unit exists — verify what the first call succeeding and every later call failing tells you.
3. When the verb declines, who notices? A boolean-returning verb that fails silently turns into a downstream state bug: the caller deletes its own bookkeeping anyway, and the UI renders a "missing/unavailable" placeholder next to an object that never went away. Audit every call site for the "fire, ignore the result, clean up state anyway" shape.

Symptom families and where they point

• First interaction works, every subsequent one errors — state written by the earlier call changed the mapping between what the plugin computes and what the verb expects. Compare the two representations before and after one insertion; derive the correction from the unit widths in the host source, then apply the same derivation at every call site that passes offsets, not just the crashing one.

• A removal button leaves the row behind with a placeholder label — the removal verb declined (see question 3) while bookkeeping was already dropped. Confirm with the verb's return value, and only retire the bookkeeping after the removal actually applied.

• Derived UI shows stale or phantom entries — find the authoritative source of the fact and derive the view from it. A plugin-side cache with a subscription that retires entries on any transient snapshot (an empty moment during reconcile/remount) will drop live entries; prefer reading the live published state at decision time and treat the cache as an accelerator only.

• Update/version chips report a wrong "latest" — remote tag and raw-file endpoints are CDN-cached and lag minutes behind a real push. Never present a fetched remote value as ground truth when it can be older than the running build; decide "current vs update" against the running version and display the newer of the two.

• A whole slot's UI silently vanishes after a release — a throwing expression inside a slot component (classically a dangling identifier: another component's state variable referenced out of scope) is caught by the framework's slot-level error boundary, which unmounts the entire entry; the error is console-only, so users just report "the chips/panel are gone". Two latency mechanisms hide it from the author: an || short-circuit keeps the expression unevaluated until the left operand is false, and components that early-return on the empty state never evaluate it until real data renders. Do not blame the newest diff by default — bisect by rollback or a minimal render mount with data present, check whether the throwing line shipped earlier, and fix by removing the reference (scope any such state locally). Cheap hardening for slot components: defensive reads (x?.items ?? []) and optional-chained DOM access (target.closest?.()) — inside an error boundary any throw costs the whole slot.

• Repo edits never reach the GUI / EBUSY under the profile's node_modules — first determine the install mode: Get-Item <profile>/node_modules/<pkg> | Select LinkType, Target (or the link:<path> marker in cordis.patch.yml). A Junction/link install means the repo working tree IS the installed copy — no copy step exists or is needed, and Copy-Item into node_modules is a no-op at best. The EBUSY holder is the running dsh host process (closing the browser does not release it), and the browser can still serve a cached client bundle after the host restarts. Activation for a link-installed lib-only plugin: fully stop the host, restart dsh web, hard-refresh, then verify the loaded version marker. Never rename-aside files under an unresolved path: through a junction "two" directories are one, and the rename moves the only copy. A source-launched host (pnpm dsh web in the harness checkout) adds two constraints: the client bundle combo is assembled once at boot (no HMR rebuilds it — every plugin edit needs a full host restart), and on Windows the listener port stays bound by the dying tree unless you stop the whole process tree (taskkill /PID <pid> /T /F), or the next boot dies on EADDRINUSE.

• One plugin with raw ESM in its client bundle takes every plugin down, and the error names an innocent entry — the host concatenates all client bundles into one classic <script> combo; a single top-level import anywhere makes the whole multi-megabyte combo fail to compile, zero plugins register, and the browser surfaces failed to import loader entry <first-entry> — the first awaited entry (often dsh-typert-registry, itself perfectly fine), not the culprit. Do not chase the named entry: bisect the profile's insert rows (or point the suspect bundle through node --check) until the combo loads again. The client half is not a bare ESM module — it must register through window.__ModuleLoader__.load({ id, factory }), pull react inside the factory via require("react"), and export inject/apply.

• Phantom pixels at the right edges of a terminal sprite (outline, Z symbols, hearts), and ghost pixels surviving frame switches — two half-block ANSI rendering defects, both invisible in the frame data: (a) a half-filled cell (upper-half block with only one half colored) sets the foreground but leaves the SGR background from the PREVIOUS cell set — SGR persists across cells, so the stale background paints a phantom pixel into the empty half; reset it explicitly (ESC[49m on every half-filled cell). (b) rows trimmed at their trailing transparent cells let a NARROWER frame leave the previous frame's pixels to the right of the trim — paint every row across the full sprite width (transparent cells as plain spaces) and close with an erase-to-EOL (ESC[K). And pin the frame data itself: hand-ported sprite frames drift from the source art a few cells at a time (a regression over an excerpt misses it) — digest every frame against the source and assert the digests.

• A head/UI process hangs for minutes (or until the CI timeout) after its work is done — a rescheduling timer chain (an animation planner that re-arms setTimeout forever while mounted) keeps the event loop alive on hosts that mount the component without ever unmounting it (probe and test hosts; a GitHub job defaults to a 6h timeout). The interactive TUI stays alive on its TTY/stdin handles regardless — so unref the chain (timer.unref()): the probes drain and exit, real sessions lose nothing. Suspect this whenever enabling a feature flips previously-finishing jobs into timeouts.

Workflow

1. Reproduce once and capture the exact user-visible strings (toast text, chip labels, console output) — they are the contract of the bug report.
2. Map each string to the code path that emitted it; identify the host verb at the boundary.
3. Open the host source for that verb; answer the three standing questions.
4. State the mismatch precisely (which representation, which guard, which call sites) before writing any fix; if you cannot state it, you have not read enough source.
5. Fix every call site that passes representation-dependent values, not only the reported symptom; the same mismatch usually breaks two features through two different verbs.
6. Prove the fix with the interaction sequence that failed: repeat the action twice in a row and assert both attempts behave identically, and assert the removal path clears every view of the object.
7. For lib-only plugin bundles (no build step): keep hand-inlined version constants in sync with package.json, syntax-check the bundle (node --check), and verify in the browser after a hard refresh — the served artifact is the file you edited.