coverage-v8: Duplicate statements when merging coverage from multiple projects with different environments

Describe the bug

When using Vitest projects with different environments (e.g., jsdom for unit tests + browser for component tests), files covered by multiple projects have duplicate coverage entries, resulting in inflated statement/function/branch counts.

Reproduction

https://github.com/stevez/vitest-coverage-merge-bug

git clone https://github.com/stevez/vitest-coverage-merge-bug
cd vitest-coverage-merge-bug
npm install
npm run test:coverage
node check-coverage.cjs

Output shows duplicate statements for constants.ts:

File: constants.ts
  Statements: 6
    [0] line 1:22 -> 7:null      <- from jsdom
    [1] line 9:21 -> 9:null      <- from jsdom
    [2] line 11:22 -> 15:null    <- from jsdom
    [3] line 1:22 -> 7:10        <- from browser (duplicate!)
    [4] line 9:21 -> 9:58        <- from browser (duplicate!)
    [5] line 11:22 -> 15:10      <- from browser (duplicate!)

Expected: 3 statements
Actual: 6 statements

Root Cause

Vite's SSR transform (used for jsdom) produces source maps with end.column: null, while browser transform produces specific end column values. Istanbul's merge() treats these as different statements because the locations don't match exactly.

System Info

System:
  OS: Windows 11
  CPU: Intel(R) Core(TM) Ultra 9 285K
  Memory: 64 GB
  Node: v22.16.0
Binaries:
  npm: 10.9.2
npmPackages:
  vitest: ^4.0.16
  @vitest/coverage-v8: ^4.0.16
  @vitest/browser: ^4.0.16

Used Package Manager

npm

Validations

• Check that you are using the latest version of Vitest
• Read the contribution guide
• Check existing issues for duplicates

Run the linked reproduction with the listed npm commands and inspect check-coverage.cjs to confirm the duplicate locations. Then trace the coverage-v8 merge path and Istanbul's merge handling for the jsdom and browser source-map locations. Done means constants.ts reports three statements rather than six while preserving coverage from both environments.