Layout Shift Guard
Stop content jumping as a page loads, and catch it when it starts happening again.
What it adds
Reserved space for everything that arrives late, plus measurement that reports unexpected movement on the app's main screens.
What your agent is told to do
5
What your agent is told to do
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Walk the app's main screens with the network throttled and note everything that moves after first paint. Images, avatars, embeds, banners, ad slots, and asynchronously loaded panels are the usual sources.
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Give every image, video, iframe, and embed explicit dimensions or a fixed aspect ratio so its box exists before its content arrives, including the ones whose size comes from data.
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Control the swap from fallback to web font so the two have compatible metrics, and accept a slightly imperfect fallback over a page that reflows once the real font lands.
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Reserve space for anything conditional that appears above or beside existing content — validation messages, incident banners, trial notices, unsaved indicators — rather than letting it push the layout down when it arrives.
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Do not count movement the user asked for. Expanding a row, opening a drawer, or loading the next page on a click is intended motion, and a guard that flags it will be turned off within a week.
Edge cases it handles
7
Edge cases it handles
7- Media and embeds without declared dimensions collapse to nothing and then snap to full size; every one needs a reserved box, including those sized from record data.
- A fallback font with different metrics to the real one reflows every paragraph when the swap happens, so the two must be matched or the swap must be suppressed until the real font is ready.
- Banners, toasts, and validation messages are the most common late arrivals in an app that has otherwise been optimised, and each needs its space held open in advance.
- Movement caused directly by a user action must be excluded from measurement, or the signal is drowned in noise and stops being actionable.
- Sticky headers and action bars that change height on scroll will register as shifts unless their reserved height is fixed; this overlaps Sticky Action Bar, which owns the pinning behaviour while this entry owns the space reservation.
- A skeleton placeholder must match the real content's dimensions. A skeleton that is the wrong height causes the shift it was meant to prevent.
- Measurement must run on the devices and connections real users have, not only on a fast machine where everything arrives before first paint.
Definition of done
8
Definition of done
8- Every image, video, iframe, and embed on the main screens has reserved dimensions before its content loads.
- Font swapping produces no visible reflow of surrounding text.
- Conditional banners and validation messages appear in space already held for them.
- Layout movement is measured on the app's main screens and reported.
- Movement resulting from a direct user action is excluded from the measurement.
- Skeleton placeholders match the dimensions of the content that replaces them.
- The feature matches the existing design system.
- No existing functionality is broken.
Related features
Font Subsetting
Font Subsetting
Load only the glyphs a page needs, without breaking the alphabets it did not expect.
What it does
Per-script font subsets with declared character ranges and a fallback order for characters no subset covers.
How it works
- 1 Inventory the weights, styles, and families the app genuinely renders. Most projects ship two or three weights they never use, and dropping those is a larger win than any subsetting.
- 2 Split each family by writing system and declare the character ranges each file covers, so a page that renders only Latin text never fetches the Cyrillic or Greek file.
- 3 Choose subsets from the locales the product supports and the languages its users actually write in, not from the words currently on the page.
Copy the prompt
No account needed
Add this feature to my app:
https://addthisfeature.com/x/font-subsetting
Component Render Profiler
Component Render Profiler
Show which components re-render, what triggered it, and which of those actually cost anything.
What it does
A development-only instrument that records render counts, their causes, and their durations for the app's live screens.
How it works
- 1 Instrument the app's real screens rather than a test harness. Re-render problems come from the shape of actual data and actual state, and they do not reproduce on three sample rows.
- 2 Record for each render what changed to cause it, distinguishing a changed input, a state update in the component itself, and a shared value that changed above it.
- 3 Report duration alongside count, and sort by total time spent. A component that renders two hundred times cheaply matters less than one that renders four times and blocks the main thread each time.
Copy the prompt
No account needed
Add this feature to my app:
https://addthisfeature.com/x/component-render-profiler
Render Stability Guardrails
Render Stability Guardrails
Make render optimisation provable, so it removes work instead of introducing stale UI.
What it does
A set of rules and checks governing memoisation, keys, and render boundaries on the app's busiest views.
How it works
- 1 Profile a real interaction on a real view before changing anything, and write down the cost being removed. An optimisation with no measured baseline cannot be shown to have worked and cannot be safely reverted.
- 2 Require every cached callback and derived value to declare each value it reads, and treat a missing declaration as a defect — a handler holding last render's values will act on data the user can no longer see.
- 3 Derive list keys from the stable identity of the record. A position-based key silently reuses the wrong element as soon as the list is sorted, filtered, or has an item removed from the middle.
Copy the prompt
No account needed
Add this feature to my app:
https://addthisfeature.com/x/render-stability-guardrails
How it works
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1
Copy the link
Grab the Markdown instruction URL for this feature.
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Give it to your AI
Paste it into Claude Code, Cursor, v0, Lovable — whatever you build with.
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It inspects, then implements
Your agent reads your existing app first, then adds the feature to fit it.
Works with your stack
These instructions are written to adapt. They tell the agent to detect your framework, match your existing design system, and reuse what you already have — rather than assuming a particular stack.
Need it tighter than that? Customize the feature and tell it exactly what you're running.