Closes #5710

Summary

Hiding the sidebar places the paragraph button near the left edge of the window. Opening its centered menu in an 800px-wide Electron window positioned the menu at x = -56px, cutting off its left side.

Add Floating UI shift middleware after offset and flip so shared floating menus stay within the window with 8px of padding. Add an Electron regression test that hides the sidebar, checks both horizontal menu boundaries, and converts the paragraph to a heading through the menu.

Type of change

• Bug fix (non-breaking, fixes an issue)
• New feature (non-breaking, adds functionality)
• Breaking change (causes existing functionality to change)
• Documentation update

Test plan

• New tests added
• Manually tested on: automated Electron tests run on macOS

Validation:

• Reproduced before the fix: regression test failed with menu x = -56px.
• After the fix: paragraph-menu regression and layout-toggle E2E specs passed (5 tests).
• BaseFloat hide-race unit tests passed (2 tests).
• pnpm run build:unpack, pnpm run lint, and pnpm run typecheck passed.
• Muya lint and type checks passed. Lint reports existing warnings only.

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.