Add Plus Button (1/3): TabbedPane trailing-button slot + overflow event

Preparatory CL for the Plus Button feature. Adds a declarative API on
TabbedPane so a follow-up can plug in a "+" / "More tools" affordance as
a Web Component. No user-visible change; existing callers keep the
legacy chevron behavior.

New APIs:
  - <slot name="trailing-button"> in the header. When assigned,
    the legacy chevron is suppressed and the slotted element's
    width is reserved during overflow detection so it stays
    visible even on the last tab.
  - "overflow-tabs-changed" CustomEvent (bubbles, composed) with
    {hiddenTabs: [{id, title, jslogContext?}]}, debounced against
    the last-dispatched set.
  - hiddenTabs() / firstHiddenTabIndex() for synchronous reads.
  - moveTab(tabId, newIndex) reorders a tab even when overflowed
    and dispatches TabOrderChanged. `newIndex` is the final
    position, clamped to [0, length-1].

A single ResizeObserver on headerContentsElement re-runs overflow
detection when the tabs container shrinks (e.g. the issues counter
loading into the right toolbar). Visible-row DOM order is reconciled
declaratively in updateTabsDropDown().

Prototype CL:
https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/7792390

Bug: 506004822
Change-Id: Ife9bd88185a413ec565e8e9593d8152e5aeb92e7
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/7814020
Commit-Queue: Guangyue Xu <[email protected]>
Reviewed-by: Danil Somsikov <[email protected]>
2 files changed
tree: 9a69dac4aabe1d874399121847839245d6e6b489
  1. .agents/
  2. .gemini/
  3. .github/
  4. .vscode/
  5. build_overrides/
  6. config/
  7. docs/
  8. extension-api/
  9. extensions/
  10. front_end/
  11. inspector_overlay/
  12. mcp/
  13. node_modules/
  14. scripts/
  15. test/
  16. third_party/
  17. v8/
  18. .clang-format
  19. .clang-format-ignore
  20. .editorconfig
  21. .env.template
  22. .geminiignore
  23. .git-blame-ignore-revs
  24. .gitallowed
  25. .gitattributes
  26. .gitignore
  27. .gitmodules
  28. .gn
  29. .mailmap
  30. .npmignore
  31. .npmrc
  32. .style.yapf
  33. .stylelintignore
  34. .stylelintrc.json
  35. AUTHORS
  36. BUILD.gn
  37. codereview.settings
  38. CONTRIBUTING.md
  39. DEPS
  40. DIR_METADATA
  41. eslint.config.mjs
  42. favicon.ico
  43. LICENSE
  44. OWNERS
  45. package-lock.json
  46. package.json
  47. PRESUBMIT.py
  48. README.md
  49. WATCHLISTS
README.md

Chrome DevTools frontend

npm package

The client-side of the Chrome DevTools, including all TypeScript & CSS to run the DevTools webapp.

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The frontend is available on chromium.googlesource.com. Check out the Chromium DevTools documentation for instructions to set up, use, and maintain a DevTools front-end checkout, as well as design guidelines, and architectural documentation.

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