diff --git a/frontend/src/components/rules/RuleEditorSlideOver.vue b/frontend/src/components/rules/RuleEditorSlideOver.vue
index a6149082..530cfd9a 100644
--- a/frontend/src/components/rules/RuleEditorSlideOver.vue
+++ b/frontend/src/components/rules/RuleEditorSlideOver.vue
@@ -414,6 +414,12 @@ watch(() => props.ruleId, load);
   </div>
 </template>
 
+<!-- `.rule-chip` for the "suggested" marker on a lesson — the chip the rule
+     panes already use, loaded here because the slide-over can open with no
+     pane that loads it (a link straight to ?rule=N).
+     Block 0, as in every pane that loads it: a shared src must sit at
+     one index everywhere (tests/test_frontend_shared_styles.py). -->
+<style src="@/assets/rules-shared.css" />
 <style scoped>
 .kind { border: 1px solid var(--fs-border-color); border-radius: var(--fs-radius-md); padding: var(--fs-space-3); margin: var(--fs-space-3) 0; }
 .kind legend { font-size: var(--fs-size-tiny); text-transform: uppercase; letter-spacing: var(--fs-tracking-tiny); color: var(--fs-text-tertiary); padding: 0 var(--fs-space-2); }
@@ -518,8 +524,11 @@ legend { padding: 0 0.35rem; font-size: 0.8rem; color: var(--fs-text-tertiary);
 .trash:hover, .close:hover { opacity: 1; }
 </style>
 
-<!-- `.rule-chip` for the "suggested" marker on a lesson — the chip the rule
-     panes already use, loaded here because the slide-over can open with no
-     pane that loads it (a link straight to ?rule=N). -->
-<style src="@/assets/rules-shared.css" />
-<style src="@/assets/moments-shared.css" />
+<!-- An @import, not a second <style src>: plugin-vue caches ONE descriptor per
+     src file, and every other importer of moments-shared.css has it as style
+     block 1. Here it would be block 2, so whichever SFC registered last decided
+     the lookup and the production build failed with "reading 'scoped'" when
+     the transform order changed (tests/test_frontend_shared_styles.py). -->
+<style>
+@import "@/assets/moments-shared.css";
+</style>
diff --git a/frontend/src/views/LessonDetailView.vue b/frontend/src/views/LessonDetailView.vue
index f9a159c4..4c5718f5 100644
--- a/frontend/src/views/LessonDetailView.vue
+++ b/frontend/src/views/LessonDetailView.vue
@@ -313,6 +313,11 @@ onMounted(load);
   </div>
 </template>
 
+<!-- `.rule-chip` and its preference variant: the marker a rule's kind wears
+     everywhere else, reused rather than re-spelled.
+     Block 0, as in every pane that loads it: a shared src must sit at
+     one index everywhere (tests/test_frontend_shared_styles.py). -->
+<style src="@/assets/rules-shared.css" />
 <style scoped>
 .lesson-detail {
   max-width: 780px;
@@ -459,7 +464,3 @@ onMounted(load);
   font-size: 0.88rem;
 }
 </style>
-
-<!-- `.rule-chip` and its preference variant: the marker a rule's kind wears
-     everywhere else, reused rather than re-spelled. -->
-<style src="@/assets/rules-shared.css" />
diff --git a/frontend/src/views/LessonEditorView.vue b/frontend/src/views/LessonEditorView.vue
index 475b5389..f3e47816 100644
--- a/frontend/src/views/LessonEditorView.vue
+++ b/frontend/src/views/LessonEditorView.vue
@@ -459,6 +459,11 @@ onMounted(() => {
   </div>
 </template>
 
+<!-- `.rule-chip-preference` marks a preference among the rules offered, as it
+     does everywhere else a rule's kind is shown.
+     Block 0, as in every pane that loads it: a shared src must sit at
+     one index everywhere (tests/test_frontend_shared_styles.py). -->
+<style src="@/assets/rules-shared.css" />
 <style scoped>
 .lesson-editor {
   max-width: 820px;
@@ -606,7 +611,3 @@ onMounted(() => {
 .le-small { padding: 0.2rem 0.7rem; font-size: var(--fs-size-label); }
 .le-ghost:disabled { opacity: var(--fs-disabled-opacity); cursor: default; }
 </style>
-
-<!-- `.rule-chip-preference` marks a preference among the rules offered, as it
-     does everywhere else a rule's kind is shown. -->
-<style src="@/assets/rules-shared.css" />
diff --git a/tests/test_frontend_shared_styles.py b/tests/test_frontend_shared_styles.py
new file mode 100644
index 00000000..2c47b316
--- /dev/null
+++ b/tests/test_frontend_shared_styles.py
@@ -0,0 +1,53 @@
+"""A stylesheet shared by `<style src>` must sit at the same block index in
+every component that loads it that way.
+
+plugin-vue caches one SFC descriptor per `src` file and resolves a request for
+`moments-shared.css?vue&type=style&index=N` against whichever component last
+registered it. When two components carry the same src at different indexes,
+the lookup can land on a component with fewer style blocks, and the production
+build dies with "Cannot read properties of undefined (reading 'scoped')". It
+depends on transform order, so an unrelated change, such as a new import in a
+view, is enough to flip it. vue-tsc cannot see it; only `vite build` can, and
+that runs at image-build time, after every test has passed.
+"""
+from __future__ import annotations
+
+import pathlib
+import re
+from collections import defaultdict
+
+ROOT = pathlib.Path(__file__).resolve().parents[1] / "frontend" / "src"
+STYLE_TAG = re.compile(r"<style\b([^>]*)>")
+SRC_ATTR = re.compile(r'\bsrc="([^"]+)"')
+
+
+def src_indexes(sources: dict[str, str]) -> dict[str, dict[int, list[str]]]:
+    """{src: {block index: [component, ...]}} over a set of SFC sources."""
+    seen: dict[str, dict[int, list[str]]] = defaultdict(lambda: defaultdict(list))
+    for name, text in sources.items():
+        for index, attrs in enumerate(STYLE_TAG.findall(text)):
+            match = SRC_ATTR.search(attrs)
+            if match:
+                seen[match.group(1)][index].append(name)
+    return seen
+
+
+def disagreements(sources: dict[str, str]) -> dict[str, dict[int, list[str]]]:
+    return {src: dict(at) for src, at in src_indexes(sources).items() if len(at) > 1}
+
+
+def test_every_shared_style_src_sits_at_one_block_index():
+    sources = {str(p.relative_to(ROOT)): p.read_text() for p in ROOT.rglob("*.vue")}
+    assert sources, f"no .vue files under {ROOT}"
+    assert disagreements(sources) == {}, (
+        "a <style src> sheet sits at different block indexes across components — "
+        "load it with @import in the odd one out"
+    )
+
+
+def test_the_guard_can_fail():
+    sources = {
+        "A.vue": '<style scoped></style>\n<style src="@/x.css" />',
+        "B.vue": '<style scoped></style>\n<style src="@/y.css" />\n<style src="@/x.css" />',
+    }
+    assert disagreements(sources) == {"@/x.css": {1: ["A.vue"], 2: ["B.vue"]}}