Browser mode: coverage-v8 leaks memory between tests #11447

Describe the bug

AI disclaimer: the reproduction was done using Claude, but the problem was very much confirmed by our CI/my laptop regularly running out of memory during full test suite runs

Might be related to #10810, but it's not the exact same symptom, so I wanted to report it either way.

In browser mode with coverage.provider: 'v8', the JS realm of every finished test file stays alive until the tab is closed. Each file adds its entire object graph (modules, DOM, closures) to the tab's heap, so a tab that runs many files eventually runs out of memory. In our suite (83 files, 12 tabs) tabs reached the --max-old-space-size cap and the run failed with [birpc] rpc is closed, cannot call "createTesters" / Browser connection was closed … Was the page closed unexpectedly?. Raising the heap size only delays the crash.

The leak doesn't happen without coverage, or with @vitest/coverage-istanbul.

You can find more info in the reproduction repo.

Reproduction

https://github.com/SomaH123/vitest-v8-coverage-leak-repro

The linked repo runs 8 tiny test files in one tab (fileParallelism: false, isolate: true). Each keeps a ~10 MB object graph on its own globalThis. A browser command forces a GC through CDP at the start of each file and prints the heap size and the number of live documents:

no coverage          5 MB/2  5 MB/2  5 MB/2  5 MB/2  5 MB/2  5 MB/2  5 MB/2  5 MB/2
coverage-v8          5 MB/2  13 MB/3  22 MB/4  31 MB/5  40 MB/6  49 MB/7  57 MB/8  66 MB/9
coverage-istanbul    5 MB/2  5 MB/2  5 MB/2  5 MB/2  5 MB/2  5 MB/2  5 MB/2  5 MB/2
(heap in MB / live documents)

With V8 coverage each finished file leaves one document and its heap behind. Without it, the removed iframe is collected.

System Info

System:
    OS: Linux 6.18 Manjaro Linux
    CPU: (16) x64 11th Gen Intel(R) Core(TM) i7-11850H @ 2.50GHz
    Memory: 11.47 GB / 31.07 GB
    Container: Yes
    Shell: 5.3.9 - /bin/bash
  Binaries:
    Node: 24.19.0 - /home/somah/.nvm/versions/node/v24.19.0/bin/node
    npm: 12.0.2 - /home/somah/.nvm/versions/node/v24.19.0/bin/npm
    pnpm: 10.33.0 - /usr/bin/pnpm
  Browsers:
    Brave Browser: 147.1.89.145
    Chromium: 148.0.7778.96
    Firefox: 150.0.1
    Firefox Developer Edition: 150.0.1
  npmPackages:
    @vitest/browser-playwright: 5.0.1 => 5.0.1 
    @vitest/coverage-istanbul: 5.0.1 => 5.0.1 
    @vitest/coverage-v8: 5.0.1 => 5.0.1 
    playwright: 1.63.0 => 1.63.0 
    vitest: 5.0.1 => 5.0.1

Used Package Manager

npm

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