elielvipata · Oct 23, 2026
diff --git a/.github/instructions/css-best-practices.instructions.md b/.github/instructions/css-best-practices.instructions.md
@@ -8,4 +8,4 @@ applyTo: "**/*.css"
 ## Selectors
 
 - Avoid `:has()` selectors. Because their result depends on descendant state, DOM mutations can invalidate styles on ancestors and cause expensive style recalculation, especially when selectors are broadly scoped. Instead, represent the state explicitly with a class or data attribute on the smallest container you own, and scope selectors to that marker. Add and remove the marker together with the state it represents.
-- Never match the `class` attribute by substring (`[class*="…"]`, `[class^="…"]`, `[class$="…"]`). A single such selector anywhere in the workbench stylesheet defeats Blink's per-class invalidation: every `classList` change then forces a style recalculation for that element, even when no rule references the class that changed. Measured on a 3.7k-node workbench, the ten `[class*="monaco-decoration-itemColor"]` selectors in the Modern UI tab styles alone made a full style recalculation 2.4x slower. When a class carries a generated suffix, have the code that applies it also set a stable marker class (see `DECORATION_LABEL_COLOR_CLASS`) and match that instead.
+- Never add a new match on the `class` attribute by substring (`[class*="…"]`, `[class^="…"]`, `[class$="…"]`). A single such selector anywhere in the workbench stylesheet defeats Blink's per-class invalidation: every `classList` change then forces a style recalculation for that element, even when no rule references the class that changed. Measured on a 3.7k-node workbench, the ten `[class*="monaco-decoration-itemColor"]` selectors in the Modern UI tab styles alone made a full style recalculation 2.4x slower. When a class carries a generated suffix, have the code that applies it also set a stable marker class (see `DECORATION_LABEL_COLOR_CLASS`) and match that instead. Existing `codicon-*` substring selectors are a grandfathered compatibility contract and must only be changed in a dedicated, separately validated codicon migration.
diff --git a/.github/skills/agent-host-e2e-tests/SKILL.md b/.github/skills/agent-host-e2e-tests/SKILL.md
@@ -1,6 +1,6 @@
 ---
 name: agent-host-e2e-tests
-description: Use when writing, recording, updating, or troubleshooting the agent host end-to-end tests under src/vs/platform/agentHost/test/node/e2e (black-box tests that drive the whole agent host over the AHP protocol, using a CapiReplayProxy record/replay system for Claude/Copilot/Codex). Covers adding a cross-provider test, re-recording fixtures after an SDK bump, gating non-deterministic or platform-specific tests, and diagnosing replay cache misses.
+description: Use when writing, recording, updating, validating, or troubleshooting the agent host end-to-end tests under src/vs/platform/agentHost/test/node/e2e (black-box tests that drive the whole agent host over the AHP protocol, using a CapiReplayProxy record/replay system for Claude/Copilot/Codex). Covers adding a cross-provider test, re-recording fixtures after an SDK bump, cross-platform Azure validation, gating non-deterministic or platform-specific tests, and diagnosing replay cache misses.
 ---
 
 # Agent host end-to-end tests
@@ -27,6 +27,8 @@ It documents the mental model, the fixture format, every config flag, and a symp
 2. Keep the prompt minimal and deterministic (fewer model turns → smaller, more robust fixtures).
 3. Record fixtures for every enabled provider (Workflow B). Host-only tests need no per-test recording: the shared empty fixture remains strict and fails on any model request.
 4. **Review the diff** (Workflow B step 3), then run the test in plain replay mode to confirm it's green, then commit the test + fixtures together.
+5. Run the full deterministic suite and coverage workflow described in the E2E README.
+6. Open or update a draft PR, then complete the cross-platform Azure validation in Workflow D before considering the tests ready to merge.
 
 Provider-specific assertions go in that provider's `*.integrationTest.ts` after the `defineAgentHostE2ETests(config)` call.
 
@@ -57,6 +59,19 @@ Real-time streaming, mid-turn aborts, and POSIX-specific local execution (shell
 
 Always add a comment explaining *why* the gate exists. Also add or update the corresponding entry in `e2e/KNOWN_ISSUES.md`. When the variant is enabled again, remove or update the entry in the same change.
 
+## Workflow D — Cross-platform Azure validation
+
+New Agent Host E2E tests are not ready to merge after local replay alone. Push the branch, open or update a draft PR, then use the `azure-pipelines` skill to validate the real packaged Electron integration-test path.
+
+1. Queue VS Code pipeline definition `111` with `VSCODE_BUILD_TYPE=CI`; enable Windows, Linux, and macOS x64 while disabling publishing, release, Web, ARM, Alpine, and Snap artifacts. The `azure-pipelines` skill contains the canonical command.
+2. Monitor jobs as they finish. Inspect a failed platform's Electron integration-test task immediately rather than waiting for unrelated stages to complete.
+3. Treat the Agent Host E2E result as accepted only when the Electron integration tests succeed on Windows, Linux, and macOS.
+4. Rerun an apparently unrelated or pre-existing failure in isolation before attributing it to the PR.
+5. After a platform-specific fix, rerun at least that platform. Rerun all three platforms when the fix can affect shared behavior, provider fixtures, process lifecycle, or cross-platform paths.
+6. Cancel obsolete builds after pushing a replacement commit.
+
+For additions involving timing, filesystem watching, process lifecycle, worktrees, reconnect/restart, or other known flake surfaces, require **two clean executions of every new test on each supported platform** before merge. A full three-platform build plus a targeted second build is sufficient when the second build runs the relevant tests on all affected platforms.
+
 ## Verifying & troubleshooting
 
 - Run a single provider in replay: `./scripts/test-integration.sh --run <path>` (no env var).

diff --git a/.github/skills/auto-perf-optimize/scratchpad/README.md b/.github/skills/auto-perf-optimize/scratchpad/README.md
@@ -49,7 +49,9 @@ These are reusable, generic runners. Use them directly or as templates:
 - **`chat-session-switch-smoke.mts`** — Creates multiple chat sessions with
   different content, then repeatedly switches between them via the sessions
   sidebar. Measures per-switch memory growth.
-
+- **`workbench-css-performance.mts`** — Measures Modern UI style and layout
+  cost while resizing, opening/switching/closing editor tabs, and toggling
+  workbench parts. Writes per-round data and medians to `summary.json`.
 - **`userDataProfile.mts`** — Utility for managing user-data profiles in
   smoke test runs.