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2026-05-16 | 🔧 Word Meter — Fixing the Diagnostics Drawer Rerender Bug 🐛

🐛 The Bug

🎤 The Word Meter PureScript build includes a diagnostics drawer — a collapsible panel that shows every event the app has recorded, from session starts and stops to recognition errors and wake lock transitions.

🔁 A subtle but frustrating bug: every time the word count updated, the diagnostics drawer snapped shut. 🙈 If you opened it to inspect what the app was doing and then spoke a word, it would slam closed. 😤 You would have to re-open it to see the new entries — only to have it close again on the next word.

🔍 The bug was consistently reproducible and clearly tied to state updates rather than any user interaction.

🔬 Root Cause Analysis — Five Whys

🤔 Why does the drawer close on every state update?

🔄 Because the mount function in Vdom.purs calls removeAllChildrenFromElement on the host element and then recreates the entire DOM tree from scratch on every dispatch cycle.

🤔 Why does recreating the DOM tree close the drawer?

📭 Because the details HTML element is recreated fresh each time, without an open attribute. The browser’s native open or closed state for a details element lives on the DOM node itself as an attribute, and that node is thrown away and replaced on every rerender.

🤔 Why doesn’t the new details element have the open attribute?

🪡 Because buildDiagnostics in Recording.purs always renders the details element with the same fixed set of attributes — only the test identifier attribute, no open. The view function had no way to know whether the user had opened the drawer.

🤔 Why doesn’t the view function know whether the drawer is open?

🧠 Because there was no diagnosticsDrawerOpen field in the Session record, no SetDiagnosticsDrawerOpen action in the Action sum type, and no click listener wired up to the summary element to dispatch anything when the user tapped it.

🤔 Why wasn’t this tracked in the first place?

🌐 Because the native details and summary element pair provides open and close behavior for free in static HTML — the browser handles it without JavaScript. That approach works perfectly when the DOM is stable, but it is fundamentally incompatible with our full-DOM-replacement render strategy. Every rerender discards all browser-native transient state, including the open attribute on details elements.

🛠️ The Fix

🧩 The fix follows our architecture’s first principle: if the view must reflect a piece of interactive state across rerenders, that state must live in the reducer.

📋 Changes at a glance

🏗️ Five files changed, all surgical and minimal.

🗂️ In Recording.purs, the Session record gained a diagnosticsDrawerOpen field of type Boolean, initialized to false in initialSession. The Action sum type gained a SetDiagnosticsDrawerOpen Boolean constructor, and the reduce function gained a matching case that simply updates the field. The Handlers record gained a requestToggleDiagnosticsDrawer callback. The buildDiagnostics view function was updated in two places: the details element now conditionally receives an open attribute when session.diagnosticsDrawerOpen is true, and the summary element now has a click listener that calls handlers.requestToggleDiagnosticsDrawer.

🔧 In Main.purs, the ClickHandlers record gained the matching requestToggleDiagnosticsDrawer field. The initial placeholder value for the handler reference is a no-op effect. A new handleToggleDiagnosticsDrawer function reads the current session, reads the current value of diagnosticsDrawerOpen, and dispatches SetDiagnosticsDrawerOpen with the negated value. The persistAfterAction exhaustive pattern match gained a case for SetDiagnosticsDrawerOpen that is a no-op, since drawer state is ephemeral UI state that should not be persisted across page loads. The TestHook.install call was updated to pass the new handler.

🧪 In TestHook.purs and TestHook.js, two new entries were added to the test hook surface: getDiagnosticsDrawerOpen reads the session field directly, and toggleDiagnosticsDrawer calls requestToggleDiagnosticsDrawer.

🔴🟢 Test-Driven Development

🧪 Following the red-green cycle, the failing tests were written before the fix was applied.

🔴 PureScript unit tests (red step)

🧮 A new runDiagnosticsDrawerReducerTests function was added to Test.Main. It asserts:

• 🔒 initialSession.diagnosticsDrawerOpen is false
• ✅ SetDiagnosticsDrawerOpen true sets the field to true
• ❌ SetDiagnosticsDrawerOpen false sets the field back to false
• 🔄 Toggle preserves the drawer open state across a rerender
• ⏱️ Tick preserves the drawer open state
• 🔁 Reset closes the drawer by returning to initial session

🔴 End-to-end regression test (red step)

🎭 A new Playwright test was added to the slice five diagnostics describe block:

• 🪟 Open the diagnostics drawer by clicking the summary element
• ✅ Assert the drawer has the open attribute
• 🔢 Inject a word-count update through the test hook (start, transcript, stop)
• ✅ Assert the drawer still has the open attribute after the rerenders

🔬 A second new test exercises the test hook surface:

• 📊 Assert getDiagnosticsDrawerOpen returns false before any interaction
• 🔄 Call toggleDiagnosticsDrawer and assert it returns true
• 🔄 Call toggleDiagnosticsDrawer again and assert it returns false

🟢 Green step

🏗️ After applying all the changes and rebuilding the bundle with npm run build:ps, all 58 end-to-end tests passed and all PureScript unit tests passed.

🧠 Design Decisions

🤷 Why not use a smarter vdom diff instead?

🔄 A virtual DOM diffing and patching approach would preserve DOM node identity across rerenders and would mean the browser’s native open or closed state would survive updates automatically. That is a valid long-term direction, but it is a much larger change to the entire rendering architecture. The reducer-based approach is the minimal, principled fix that matches our existing patterns — every piece of interactive state that must survive rerenders already lives in the reducer.

🤷 Why not use preventDefault on the summary click?

🚫 The click listener on the summary element calls our dispatch function, which synchronously updates the session and replaces the entire DOM subtree. By the time the browser’s default summary-click handling runs, the original details node has already been detached from the document. The browser’s default behavior operates on the detached (old) node and has no visible effect. So preventing default is unnecessary — the two paths do not conflict.

🤷 Why is diagnosticsDrawerOpen reset on Reset?

🔁 Reset returns the entire session to initialSession (with a few fields explicitly preserved: diagnostics history, environment snapshot, and keep-awake preference). Drawer state is transient UI state, not session data, so it returns to the closed default on reset. This is consistent with how the error banner and other ephemeral fields behave.

🤷 Why is it not persisted?

💾 The drawer open/closed preference is session-level UI state, not part of the counting history that the user cares about across page loads. Persisting it would add complexity for negligible benefit. The keep-awake preference also follows this convention — it is initialized to true on every page load rather than being saved.

📚 Book Recommendations

📖 Similar

• The Practice of Programming by Brian W. Kernighan and Rob Pike is relevant because it emphasizes writing clean, maintainable code and debugging systematically — exactly the skills applied here in tracing the bug through five layers of causation to its root.
• 🐞🔍🤔✅ Debugging: The 9 Indispensable Rules for Finding Even the Most Elusive Software and Hardware Problems by David J. Agans is relevant because it codifies the disciplined root-cause-analysis approach used here, where symptoms are traced methodically to their origin rather than patched at the surface.

↔️ Contrasting

• React and React Native: A Complete Hands-On Guide to Modern Web and Mobile Development by Adam Boduch contrasts this approach by advocating a framework-managed virtual DOM that preserves interactive element state automatically across re-renders, which would have prevented this class of bug from existing at all — at the cost of a heavier abstraction layer.

🔗 Related

• Programming in Haskell by Graham Hutton is related because the functional-core immutable-state approach underpinning this entire Word Meter architecture — where the reducer is a pure function from action and old state to new state — traces directly to the typed functional programming tradition Haskell exemplifies.