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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsMost Shopify checkout UI extension problems are not ordinary CSS bugs. The checkout page is a constrained host surface: an extension cannot access the host checkout DOM, override Shopify component CSS, or render arbitrary HTML. A visible-but-wrong extension therefore needs a supported component and layout fix, while a missing extension needs target, placement, layout-context, or runtime checks.
Use this order: identify the extension generation and target, separate unsupported styling expectations from real layout defects, verify placement in the editor, reproduce the merchant’s checkout layout and buyer state, inspect mount and runtime errors, then measure visibility and buyer usability separately.
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Classify the symptom before changing code
Start by recording exactly what the merchant sees. The same visual complaint can come from different causes, and each cause has a different fix.
| Observed symptom | Most likely diagnostic class | First check |
|---|---|---|
| The extension appears, but spacing, type, borders, or alignment differ from the design | Unsupported host styling or an incorrect use of supported components | Review component properties, typography, spacing, stack or grid usage, and merchant branding settings |
| The extension is absent only at one location | Target, placement, or checkout-layout behavior | Verify the target and placement, then test the exact checkout layout and buyer-data state |
| The extension is absent everywhere | Configuration, activation, mounting, or runtime failure | Inspect extension metadata, API version, editor activation, and runtime errors |
| Content eventually appears but is slow | Network wait, bundle download or parse, rendering work, or post-render main-thread work | Measure extension_visible and extension_buyer_usable on a real checkout |
| The extension is visible but cannot be used promptly | Work occurring after the first render or blocked input handling | Inspect post-render tasks and input responsiveness rather than only download time |
Keep the comparison controlled: use the same extension version, target, placement, buyer-data state, and test account when comparing two checkout layouts. Record both the expected location and the actual location; this prevents a placement difference from being mistaken for a styling regression.
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Know what the checkout surface allows
Checkout UI extensions run inside Shopify’s controlled checkout surface, not inside the merchant’s page markup. Shopify states, “You can’t override the CSS for UI components.” Extension code also does not have access to the real checkout DOM and cannot render arbitrary HTML. A selector aimed at checkout internals, an injected stylesheet, or custom markup that worked in a storefront theme is not a supported checkout-extension solution.
Redesign the visual requirement with the primitives Shopify exposes for the extension’s API generation. Use component properties for supported typography, spacing, alignment, and appearance; use sections and other documented UI components; and account for merchant branding settings that can influence the final presentation. If the requirement depends on a host selector, an element that Shopify does not expose, or a custom HTML structure, classify it as a platform constraint rather than an intermittent browser bug.
Choose stack or grid for the actual layout
The supported layout choice should match the geometry of the content:
| Primitive | Best fit | How to troubleshoot it |
|---|---|---|
| Stack | One-dimensional responsive arrangements, such as a row or column of controls | Check direction, spacing, alignment, and how the stack behaves at the available checkout width |
| Grid | Complex two-dimensional arrangements that need explicit rows and columns | Check the defined columns, item placement, and the narrow-width behavior instead of adding CSS rules |
Shopify recommends stack for one-dimensional responsive arrangements and grid for complex two-dimensional layouts. If a design only works after adding arbitrary CSS, simplify it to these supported primitives or revise the design. A merchant can also change branding options, so avoid treating a screenshot from one store as a universal rendering contract.
Confirm the extension generation, API version, and target
Before investigating pixels, establish which extension generation is running. The configured API version determines the component set and the examples that apply. Shopify identifies API version 2025-07 as the final version that supports React-based UI components; later API versions use Polaris web components. Do not mix a React example with a web-component extension simply because both snippets appear in search results.
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The checkout UI extension references displayed API version 2026-07 as the latest version at the time of the October 13, 2026 documentation check. Version labels and migration policies can change, so verify the current reference before a production upgrade. The versioned 2025-07 component documentation names October 1, 2026 as a date after which an extension that has not upgraded could be blocked from updating. That date has passed; consult the current upgrade policy before relying on the warning or planning a release.
Inspect the configuration and runtime metadata
- Open the extension configuration and record its API version and configured target.
- Confirm that the running extension reports the target you expect, rather than a target from another extension or build.
- Check the API version exposed by the extension metadata and make sure the code uses components from that generation.
- Record whether the extension is running in the checkout editor, a development checkout, or the published merchant checkout.
- Use the extension’s rendered state to distinguish code that has mounted from code that has merely been pre-rendered.
These values turn “it does not show” into a concrete question: is the expected target active, is the correct version executing, is the editor context supported, and did the extension reach a rendered state?
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Verify target type and merchant placement
Targets determine where an extension can appear. A static target has a fixed location supplied by Shopify. A block target is a merchant-positioned app block and will not appear in the published checkout until the merchant adds and places it in the checkout and accounts editor.
Static-target checks
- Confirm that the configured static target exists in the current API version.
- Verify that the test checkout is the surface associated with that target.
- Check the target reported by extension metadata while the page is running.
- Compare the expected page or section with the checkout layout being tested.
Block-target checks
- Run the local development workflow with
shopify app dev. - Open the CLI-generated checkout URL and add the extension as an app block in the checkout and accounts editor.
- Place the block at the intended supported surface, save the editor configuration, and reload the checkout.
- Preview individual placements by adding
?placement-reference={name}to the CLI-generated checkout URL. - Repeat the preview for each placement the extension claims to support, rather than assuming that one successful location proves all locations work.
Shopify lists checkout placement references including WALLETS1, INFORMATION1 through INFORMATION3, DELIVERY1 through DELIVERY2, and PAYMENT1 through PAYMENT4. Substitute the exact reference name when using the query parameter; do not add spaces or alter its capitalization.
If a block is missing in the published merchant checkout but visible during development, first compare editor activation and placement. Development rendering does not automatically place the block in a merchant’s saved checkout configuration.
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Reproduce the checkout layout and buyer state
Checkout structure changes what “the same target” means. Test at least three-page checkout, one-page checkout, and Shop Pay when those contexts are relevant to the merchant.
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| Context | Documented behavior | What to compare |
|---|---|---|
| Three-page checkout | Section-specific targets show on their corresponding page | Whether the buyer has reached the page associated with the target |
| One-page checkout | Section-specific extensions mount together because the sections are on one page | Whether extensions expected on separate steps now appear together or change order |
| Shop Pay | Uses a one-page layout; the merchant controls extension inclusion through the editor | Editor inclusion, one-page placement, and the same buyer-data conditions |
In three-page checkout, an extension can be correctly mounted but invisible because the buyer is looking at a different step. In one-page checkout, extensions that were separated across steps can be visible at the same time, so a merchant may report an apparent ordering or spacing problem even though each extension is mounted at its supported location.
Account for collapsed sections and rollups
Prefilled buyer information can collapse sections into rollups. Shopify documents different behavior for static and block extensions:
- Static extensions continue to render within the rollup.
- Block extensions belonging to collapsed sections move toward the bottom of the checkout.
Therefore, a block that “disappears” after an address or contact field is prefilled may have moved rather than failed to render. Reproduce with an empty buyer profile and with the merchant’s typical prefilled data. Capture the expanded and collapsed states in the evidence you send to the developer.
Separate mounting and runtime failures from visibility
A target can be correct while the extension fails during initialization. Conversely, the extension can execute successfully but be outside the currently visible section. Use metadata and runtime diagnostics together.
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Use supported extension state and metadata
Inspect the extension’s target, API version, editor context, and rendered state through the extension API. Shopify notes that extensions may be pre-rendered before a buyer reaches the corresponding step. Expensive work can therefore wait until the rendered state is true; doing so avoids spending resources on an extension that is not yet visible.
Do not assume ordinary page-level access. Checkout UI extensions execute in an isolated worker context without the regular checkout window object or checkout DOM. Diagnostics should use the extension runtime and its supported logging facilities, not selectors or scripts that depend on host-page internals.
Capture rejected promises, exceptions, and resource errors
Register listeners for unhandled promise rejections and runtime errors early in development so a mount failure is not reduced to a blank area:
addEventListener('unhandledrejection', event => { console.error('Unhandled promise rejection', event.reason); });
addEventListener('error', event => { console.error('Extension error', event.error || event.message); });Adapt the registration to the event APIs available in the extension runtime. Use the captured reason to distinguish a rejected data request, a JavaScript exception, and a failed resource load. If an extension downloads but never reaches a rendered state, it still consumes checkout resources; treat that as a runtime defect even when no visible error appears in the editor.
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Measure late visibility separately from late usability
Performance testing in the checkout editor is not enough. Shopify recommends measuring a real checkout under constrained conditions, because network and device characteristics change the result.
- Open a real checkout in an incognito window.
- Disable the browser cache for the test.
- Throttle the network; Shopify gives Slow 4G as an example.
- Use the Network panel to identify extension requests, waits, and failed resources.
- Record a Performance panel trace around navigation, extension mount, and buyer interaction.
- Repeat with mobile emulation or on a phone, because CPU cost can change which task is dominant.
- Compare the time at which content becomes visible,
extension_visible, with the time at which the buyer can use it,extension_buyer_usable.
A large bundle is not automatically the slowest part. Look at the observed phase instead:
| Symptom in the trace | Likely work to inspect | Potential direction |
|---|---|---|
| Visible content is late | External-data waits, bundle download or parse, and initial render | Reduce blocking work and consider a lightweight placeholder before a required fetch |
| Content is visible but interaction is delayed | Post-render JavaScript, long main-thread tasks, or input handling | Move nonessential work after interaction, split heavy work, and check responsiveness on mobile hardware |
| The extension never appears | Target or placement configuration, mount exceptions, or failed resources | Return to target, editor, metadata, and runtime-error checks |
| Only one placement is slow or absent | Placement-specific content, target support, or data requirements | Reproduce with that placement’s reference and compare its trace with a working placement |
Shopify does not publish a universal bundle-size limit for checkout UI extensions in this performance guidance. Compare the extension’s measured cost on the merchant’s own checkout instead of applying an assumed threshold.
Use a symptom-specific recovery path
When the extension is visible but styled incorrectly
- Remove assumptions about checkout selectors, inherited CSS, or arbitrary HTML.
- Identify the API generation and replace unsupported components with the matching documented components.
- Express spacing, typography, and alignment through component properties and supported layout primitives.
- Use stack for a one-dimensional arrangement and grid for a two-dimensional arrangement.
- Retest at the merchant’s checkout width and with the merchant’s branding settings.
When a block is missing
- Confirm the target in configuration and runtime metadata.
- Run
shopify app devand preview the exact placement reference. - Verify that the merchant added and placed the block in the checkout and accounts editor.
- Test the target on the correct page in three-page checkout and in the combined one-page layout.
- Check collapsed rollups and inspect runtime errors if the block remains absent.
When it appears only after a delay
- Mark the point at which the extension becomes visible and the point at which its controls accept input.
- Inspect network waits and bundle parse time before the first point.
- Inspect post-render tasks and input responsiveness between the two points.
- Repeat under Slow 4G and mobile CPU conditions.
- Optimize the phase that the trace actually identifies; do not shrink a bundle merely because it is large.
What evidence to send an extension developer
A concise evidence packet prevents multiple rounds of guessing. Include:
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- Whether the issue occurs in the editor, a development checkout, or the published merchant checkout.
- Checkout layout: three-page, one-page, or Shop Pay.
- Placement reference and whether the extension is static or block-based.
- Whether buyer information was prefilled and whether the relevant section was expanded or rolled up.
- Expected location versus actual location.
- Rendered-state observation and any target or editor metadata values.
- Console or runtime output for rejected promises, exceptions, and resource failures.
- A network and performance trace captured on a real checkout, including the network throttle and device conditions.
- The exact extension version tested and whether the result changes at another supported placement.
This information lets the developer decide whether to change the component layout, correct editor placement, handle a layout-specific mount, fix initialization, or optimize a measured bottleneck.
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