A report from Paul:

I now see we have two trace events per each FMP:

{"pid":51862,"tid":775,"ts":197784004134,"ph":"R","cat":"loading,rail,devtools.timeline","name":"firstMeaningfulPaint","args":{"frame":"E70CC022B0E6235ABB5C9A69C9E23768","data":{"navigationId":"CF77A425F5C13304AEAD7CE20E2F4712"}},"tts":2651855},
{"pid":51862,"tid":775,"ts":197784004134,"ph":"R","cat":"loading,rail,devtools.timeline","name":"firstMeaningfulPaint","args":{"frame":"E70CC022B0E6235ABB5C9A69C9E23768","afterUserInput":0},"tts":2651867},

Looks like we have a probe then we also have the paint_timing one:
1. https://cs.chromium.org/chromium/src/third_party/blink/renderer/core/paint/paint_timing.cc?l=110&rcl=78dd4267df58a40de35e49bb1e4312c31dd4d474
2. https://cs.chromium.org/chromium/src/third_party/blink/renderer/core/paint/first_meaningful_paint_detector.cc?l=271&rcl=78dd4267df58a40de35e49bb1e4312c31dd4d474

Paul, why do we need the second one? https://chromium-review.googlesource.com/c/chromium/src/+/706176/