<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Label this issue](https://contributors.gitlab.com/manage-issue?action=label&projectId=278964&issueIid=628699) </details> <!--IssueSummary end--> ## Summary After https://gitlab.com/gitlab-org/gitlab/-/merge_requests/252667 the bundler gives Vue 3 pages a Vue 3 copy of every module that is exposed to Vue. A module that both the Vue 2 global bundles and a Vue 3 page load then runs twice on that page, once per Vue version. Module-scope state in such a module becomes two objects. The duplicated-modules check in `scripts/frontend/infection_scanner/singletons.mjs` only detects known state kinds: Pinia stores, Apollo clients and reactive vars, event hubs, Vuex stores, `Vue.observable`, and `Object.defineProperty`. A bare module-scope `let`, or an empty `Map`, `Set`, array, or object, is invisible to it. The Mermaid render counter was one such case, fixed in that MR. ## Method A temporary widening of `detectSingletons` reported every module-scope `let` or `var`, every `const` initialised to an empty `Map`, `Set`, `WeakMap`, `WeakSet`, `[]`, or `{}`, and every write to a class static. The duplicated-modules check then ran with the real page seeds. Result: 24 modules load in both lanes on migrated pages and hold such state, across 190 page states. ## Findings ### Two copies are correct - `app/assets/javascripts/lib/utils/vue3compat/init_vue_app.js`, `portal_vue_vue3.js`, `vue_apollo.js`, `vue_router.js`, `vuex.js`. The state records which Vue instance the shim installed into. It must be per Vue version. - `app/assets/javascripts/tracking/internal_events.js`, `app/assets/javascripts/gl_form.js`, `app/assets/javascripts/behaviors/markdown/render_image_lightbox.js`, `app/assets/javascripts/behaviors/markdown/render_markdown_tables.js`, `app/assets/javascripts/content_editor/services/autocomplete_helper.js`. Each keys a `WeakMap` by DOM element, so two copies cannot process the same node twice. - `app/assets/javascripts/glql/components/presenters/stat.vue`, `app/assets/javascripts/members/constants.js`. Read-only lookups built once. - `app/assets/javascripts/emoji/index.js`, `app/assets/javascripts/emoji/support/index.js`. Lazy caches. Two copies compute the same value twice. Wasteful, not wrong. - Per-app or per-editor state: `app/assets/javascripts/issuable/popover/index.js`, `app/assets/javascripts/lib/utils/text_markdown.js`, `app/assets/javascripts/content_editor/services/upload_helpers.js`, `ee/app/assets/javascripts/ai/duo_agents_platform/router/utils.js`, `ee/app/assets/javascripts/ai/duo_agents_platform/utils/navigation_state.js`, `ee/app/assets/javascripts/ai/duo_agentic_chat/events/event_hub.js` (`lastProcessedMessageId` only), `ee/app/assets/javascripts/ai/editor_actions/generate_description.js`. ### Needs a decision - `app/assets/javascripts/tooltips/index.js`. `let app` guards one global tooltip Vue app mounted on `#gl-tooltips-app`. Both lanes load it on 98 page states. Two copies mean two tooltip apps, one per Vue version, both watching the document. Each lane probably wants its own app, because the tooltip component runs on its own Vue. Confirm nothing double-mounts on the same element, then add the module to `DUPLICATION_EXPECTED` in `scripts/frontend/infection_scanner/infection_scanner.mjs` with a comment. - `app/assets/javascripts/popovers/index.js`. Same pattern with `let app` and `let isInitialized`. One page state today. - `app/assets/javascripts/gfm_auto_complete.js`. The class statics on `GfmAutoComplete` are configuration, not state. Both lanes load the whole module on 53 page states. The cross-lane read it depends on, `SidebarMediator.singleton`, is kept single by the blocklist entry in that MR. ### Same guard pattern, not reached by both lanes today - `app/assets/javascripts/user_popovers.js` (`hasAddedLazyPopovers`) - `app/assets/javascripts/whats_new/index.js` (`whatsNewApp`) - `app/assets/javascripts/super_sidebar/user_counts_manager.js` (`broadcastChannel`) ## Proposal 1. Add an opt-in `BROAD_STATE_SCAN` mode to `singletons.mjs` that reports the kinds above, so this search can be repeated after each migration wave. 1. Decide on the tooltip and popover apps and record the decision in `DUPLICATION_EXPECTED`. 1. Leave the rest as is. ## References - Follow-up from https://gitlab.com/gitlab-org/gitlab/-/merge_requests/252667 - Review thread that prompted the search: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/252667#note_3819715224 - Root cause of the duplication: https://gitlab.com/gitlab-org/gitlab/-/work_items/625296 - Duplicated-modules check: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/252666 <!-- claude session=5c77459b-fe68-4804-b79d-f7368d54a6e8 posted=2026-09-11T09:10:27Z -->