Spacing on submenus seems different than in main menu items

[osortega]

Testing #334485

Feels like the spacing between the items in the main menu and the submenus is different.
Looks much more spaced out on the submenus

[vs-code-engineering]

Sharing media and context from merged PR #334908 for verification:

Summary

Keep activity-rail spacing out of nested compact application menus. The floating-rail margin selector also matched menu rows and separators, increasing normal 24px row steps to 32px at default density and 28px at compact density.

• Scope the gap selector to non-menu action bars and their own direct action items.
• Preserve the activity rail spacing and overflow calculations while restoring standard menu-row and separator spacing.
• Add real-widget regressions for default density, compact density, compact activity-bar size, and classic layout.
• Add keyboard-driven component fixtures for both densities in dark, light, and dark high contrast.

Validation

• 44 targeted tests passed across Modern UI, activity-bar sizing, Menu, and MenuBar:

./scripts/test.sh \
  --run src/vs/workbench/contrib/modernUI/test/browser/modernUI.contribution.test.ts \
  --run src/vs/workbench/test/browser/parts/activitybar/activitybarPart.test.ts \
  --run src/vs/base/test/browser/ui/menu/menu.test.ts \
  --run src/vs/base/test/browser/ui/menu/menubar.test.ts \
  --grep "ActivitybarPart|^Menu|activity bar|floating rail|compact application menu"

• The default/compact density regressions failed against the original selector. All four new regressions passed with the fix, including a repeat after baseline screenshot capture.
• All six themed fixture variants retain 24px menu-row steps with no added rail margin, while preserving the default/compact rail gaps of 8px/4px.
• Verified keyboard navigation: Undo to Redo, skipping the separator to Cut, Left/Right submenu navigation, and Escape returning focus to the application-menu button.
• ESLint, stylelint, editor diagnostics, diff checks, and pre-commit hygiene passed.

Manual verification

In the web workbench with Modern UI and the compact application menu in the activity bar, open Edit. Check normal row and separator spacing at default and compact density, then verify keyboard entry and dismissal. Repeat in light and high-contrast themes.

Menu placement and overflow reports #334616 and #334886 remain separate.

Screenshots

Captured from the same real-widget component fixture at default density, with Edit selected and Undo focused. The fix restores menu spacing without changing the rail gap.

Before	After

Source PR: #334908