Would you merge this playground to js-sourcemap playground to reduce the number of playgrounds?

Sure!

Does combineSourcemaps work without doing the same normalization (i.e. setting [this.filename]) above? This happens when a module is transformed by a plugin then transformed by another plugin that uses MagicString.

Sources are absolutely present when entering here Because the first source map has added sources or the sources themselves exist

combineSourcemaps has two inputs. The second one (combinedMap) wouldn't have sources: ['']. But the first one (m) might have sources: [''], no?

No, the second one definitely have

I've actually tested it now by changing the code like below and renaming zoo.js to zoo.ts.

          console.log('Before', m, combinedMap)
          combinedMap = combineSourcemaps(cleanUrl(this.filename), [
            m as RawSourceMap,
            combinedMap as RawSourceMap,
          ]) as SourceMap
          console.log('After', combinedMap)

I got the following result:

Before SourceMap {
  version: 3,
  file: undefined,
  sources: [ '' ],
  sourcesContent: undefined,
  names: [],
  mappings: 'AAAA,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC,CAAC;'
} {
  version: 3,
  sources: [ 'D:/documents/GitHub/vite/playground/js-sourcemap/zoo.ts' ],
  sourcesContent: [ "export const zoo: string = 'zoo'\n" ],
  mappings: 'AAAO,aAAM,MAAc;',
  names: []
}
After SourceMap {
  version: 3,
  mappings: 'AAAO,aAAM,MAAc',
  names: [],
  sourceRoot: undefined,
  sources: [ 'D:/documents/GitHub/vite/playground/js-sourcemap/zoo.ts' ],
  sourcesContent: [ "export const zoo: string = 'zoo'\n" ],
  file: 'D:/documents/GitHub/vite/playground/js-sourcemap/zoo.ts'
}

It seems it works at least for this case.