fix: prevent viewtransitions.css from loading with server:defer

[0xRozier]

Summary

Fixes withastro/compiler#1148

• The @astrojs/compiler incorrectly treats server:defer like a transition:* directive, causing viewtransitions.css to be imported on pages that use server:defer even without any ViewTransitions/ClientRouter usage.
• After compilation, strip the viewtransitions.css import from compiled output when the source doesn't contain any transition:name, transition:animate, or transition:persist directives.

Root cause

In the compiler's transform.go, server:defer is grouped with transition directives:

if HasAttr(n, TRANSITION_ANIMATE) || HasAttr(n, TRANSITION_NAME) || 
   HasAttr(n, TRANSITION_PERSIST) || HasAttr(n, SERVER_DEFER) {
    doc.Transition = true  // triggers CSS import
    doc.HeadPropagation = true
    getOrCreateTransitionScope(n, &opts, i)
}

This sets doc.Transition = true which makes the compiler emit import "astro/components/viewtransitions.css" even when only server:defer is present.

The long-term fix belongs in @astrojs/compiler, but this provides an Astro-side workaround by stripping the unnecessary CSS import post-compilation.

Test plan

• Unit tests: 4 tests verifying CSS inclusion/exclusion logic
  • CSS is not included with only server:defer
  • CSS is included with transition:name
  • CSS is included with transition:animate
  • CSS is included with both server:defer and transition:name
• Integration test: verifies rendered HTML doesn't contain viewtransitions CSS keyframes
• Build passes

🤖 Generated with Claude Code

[changeset-bot]

🦋 Changeset detected

Latest commit: 860bfeb

The changes in this PR will be included in the next version bump.

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[codspeed-hq]

Merging this PR will not alter performance

✅ 9 untouched benchmarks

---

_{Comparing 0xRozier:fix/issue-15574-viewtransitions-css-loaded-with-server-defer (860bfeb) with main (d789452)}

[Copilot]

Pull request overview

This PR fixes issue #15574 where viewtransitions.css is incorrectly loaded on pages using server:defer without any View Transition directives. The root cause lies in @astrojs/compiler, which incorrectly groups server:defer with transition directives when determining whether to import the ViewTransitions CSS. This PR implements a post-compilation workaround in Astro's compile step to strip the unnecessary CSS import.

Changes:

• Added post-compilation logic to strip viewtransitions.css import when no transition directives are present in source
• Added unit tests verifying CSS inclusion/exclusion logic for various directive combinations
• Added integration test to verify CSS is not included in rendered HTML for server:defer-only pages

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
packages/astro/src/core/compile/compile.ts	Adds post-compilation check to strip viewtransitions.css import when only server:defer is used without transition directives
packages/astro/test/units/compile/viewtransitions-css.test.js	New unit test file with 4 tests covering CSS import behavior across different directive combinations
packages/astro/test/server-islands.test.js	Adds integration test verifying viewtransitions CSS is not included in rendered HTML
.changeset/fix-viewtransitions-css-server-defer.md	Changeset documenting the patch fix

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[martrapp]

Hi @0xRozier 👋🏼 thank you for addressing the issue so fast!

I saw that you repair the unnecessary input inside the astro package after the fact.
I think it would be a more maintainable solution to address the root cause instead.

Did you have a chance take a look at the compiler repository?

[0xRozier]

Hi @martrapp, thanks for the feedback!

I agree that the compiler would be the ideal place for this fix. Unfortunately, I'm not familiar with Go and the compiler internals, so contributing a fix there isn't really something I can take on.

This workaround is intentionally scoped and defensive — it only strips the import when the source genuinely has no transition:* directives, so it should be safe as a stopgap. If someone with compiler expertise wants to address the root cause in withastro/compiler, this patch could then be removed.

Let me know if this approach works for you, or if you'd prefer to wait for a compiler-side fix instead!

[martrapp]

Hi @0xRozier, thanks for the honest reply — I really appreciate the transparency.

That makes sense, and your workaround sounds sensible as a stopgap. I will take a look at the compiler and see if I can make any progress there.

[martrapp]

Opened withastro/compiler#1149

[martrapp]

Closing this PR in favour of #1149