tapted@: Were you seeing a call to WritePackToDiskCallback? (https://cs.chromium.org/chromium/src/chrome/browser/themes/theme_service.cc?rcl=09a09494971fba3dd8ec7573abc5d0c872136303&l=98) That's pretty much the main call I can see that would write to Cached Theme.pak.

no - git was complaining that I had a modified source tree after running browser_tests. chrome/test/data/extensions/theme/Cached Theme.pak was the file that changed.

The file didn't exist before tetsui@'s change in https://chromium-review.googlesource.com/c/chromium/src/+/1086821

I don't have context for why that file was added in that cl, or why it changes.

Happens for me on Linux as well.