Add open/close animations to standard button menus + fix menu positioning

[hawkticehurst]

Related to #325020

This pull request implements open/close motion for various sidebar menus in VS Code, following the motion patterns established in previous PRs. Key changes include:

• Menu Animation: Standard button menus now inherit motion globally through the DropdownMenu, affecting sidebar/view toolbars, contributed views, SCM rows, filters, and split installs.
• Rich Dropdowns: Rich action-widget dropdowns, including the Start Debugging button, utilize shared motion via withActionWidgetDropdownMotion.
• Legacy Menus: The "Manage" menus in extensions are explicitly bridged to the new motion system.
• Accessibility: The motion is gated by the workbench.experimental.modernUI setting and respects reduced motion preferences.

New tests have been added to ensure the functionality works as expected, and all changes have been validated through type-checking and live testing.

Screen.Recording.2026-07-16.at.1.08.37.PM.mov

Additionally, fixed an issue where "Manage" icon button menus associated with installed extensions, MCP servers, and Agent plugins were anchored too far from the button.

[Copilot]

Pull request overview

Adds shared open/close motion to standard and rich dropdown menus, gated by Modern UI and reduced-motion preferences.

Changes:

• Applies shared motion defaults to DropdownMenu.
• Adds reusable motion for action-widget dropdowns.
• Migrates chat and extension menus to shared behavior and adds tests.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/vs/base/browser/ui/contextview/contextview.ts	Exposes shared motion constants and close configuration.
src/vs/base/browser/ui/dropdown/dropdown.ts	Applies motion defaults to dropdown menus.
src/vs/base/test/browser/ui/dropdown/dropdown.test.ts	Tests default and overridden motion options.
src/vs/platform/actionWidget/browser/actionWidget.css	Defines shared action-widget animations.
src/vs/platform/actionWidget/browser/actionWidgetDropdown.ts	Adds and applies shared motion options.
src/vs/platform/actionWidget/test/browser/actionWidgetDropdown.test.ts	Tests option composition and overrides.
src/vs/workbench/browser/actions/menuMotion.ts	Re-exports base motion configuration.
src/vs/workbench/contrib/chat/browser/widget/input/chatInputPickerActionItem.ts	Migrates chat pickers to shared motion.
src/vs/workbench/contrib/chat/browser/widget/media/chat.css	Removes superseded picker-specific animations.
src/vs/workbench/contrib/extensions/browser/extensionsActions.ts	Enables motion for extension management menus.