Layout Thrashing Report Analysis

Analyze a bye-thrash report and produce actionable fix suggestions for every layout thrashing instance found.

Usage

/thrash-report-analyzer <path-to-report.json>

The argument is the absolute path to a JSON file containing the bye-thrash report array. If no file path is provided, ask the user to paste the report JSON directly.

Input format

Each entry in the report array has at minimum:

• prop — the layout property that was read (e.g. offsetWidth, clientHeight)
• count — how many times this thrash was observed
• stack — a JavaScript call stack string captured at the moment of the layout read

Workflow

Follow these steps in order for each entry in the report.

Step 1 — Parse the stack trace

Split the stack string into individual frames. For each frame, extract:

• Function name (or <anonymous>)
• Source file path
• Line and column number

Ignore frames from:

• bye-thrash internals — any frame containing Thrash.onLayoutRead, onRead, or anonymous wrappers from the library's property patches
• React internals — executeDispatch, processDispatchQueue, batchedUpdates, dispatchEvent, commitRoot, performSyncWorkOnRoot, renderRootSync, workLoop, and similar scheduler/reconciler frames
• Next.js / webpack / turbopack internals — frames in next/dist, webpack-internal, turbopack, or __next

Step 2 — Identify the user-code function

Walk the filtered frames top-to-bottom. The first frame whose source file is inside the project (not in node_modules, .next, or dist) is the user-code function responsible for the thrash.

Record:

• Function name
• Source file path
• Line number

Step 3 — Search the codebase for the function

Use Grep to locate the function definition. Search .tsx, .ts, .jsx, .js files. Exclude node_modules, .next, and dist directories.

If the function name is generic (e.g. onClick, handleResize), use the source file path from the stack trace to narrow the search.

Step 4 — Read the source and find the write → read pair

Read the source file around the identified line. Look for:

Style writes (any of these before the read):

• el.style.* assignments
• classList.add(), classList.remove(), classList.toggle(), classList.replace()
• setAttribute('style', ...) or setAttribute('class', ...)
• className assignment
• innerHTML or textContent assignment (these invalidate layout)
• insertBefore, appendChild, removeChild, replaceChild

Layout reads (the property from the report's prop field):

• Dimension reads: offsetWidth, offsetHeight, offsetTop, offsetLeft, clientWidth, clientHeight, clientTop, clientLeft
• Scroll reads: scrollTop, scrollLeft, scrollWidth, scrollHeight
• Computed style: getComputedStyle(), currentStyle
• Bounding rect: getBoundingClientRect()
• Other: innerText, focus(), scrollTo(), scrollBy(), scrollIntoView()

Step 5 — Determine the fix

Choose the appropriate fix strategy:

Scenario	Fix
Read is only used for comparison/logging	Move the read before the write
Read feeds into the write value	Cache the read before the write, or use requestAnimationFrame to defer the write
Multiple elements written then read in a loop	Batch all reads first, then batch all writes
Write is visual-only and read can be deferred	Wrap the read in requestAnimationFrame
Pattern is in a resize/scroll handler	Debounce or throttle the handler; use ResizeObserver instead

Step 6 — Generate the report

Output findings in this format for each entry:

### Entry: `{prop}` (count: {count})

- **Source file:** `path/to/file.tsx`
- **Function:** `functionName` (line X)
- **Write:** `el.style.fontSize = '24px'` (line Y)
- **Read:** `window.getComputedStyle(el).fontSize` (line Z)
- **Fix suggestion:** [brief actionable suggestion]

After all entries, add a Summary section:

## Summary

- **Total thrash instances:** N
- **Unique locations:** M
- **Most frequent:** `prop` in `functionName` (K occurrences)
- **Highest impact fix:** [which fix would eliminate the most thrash instances]

Edge cases

• Stack trace points to minified code — If the source map is not available and function names are mangled, note this in the report and suggest the user provide source maps or an unminified build.
• Multiple writes before a single read — Report all writes that contribute to the invalidation, not just the closest one.
• Thrash happens inside a third-party library — If the first user-code frame is a call into a library that internally thrashes, note the library name and suggest the user file an issue or wrap the call in a requestAnimationFrame.
• No matching source found — If Grep cannot locate the function, report the stack trace as-is and suggest the user verify the source is available in the working directory.