Ian, Alex, any thoughts on who to route this to?
I'll bisect tomorrow, but if you happen to know offhand.

horo was working on this, probably https://chromium-review.googlesource.com/982776

I've bisected the issue down to this commit:
https://chromium-review.googlesource.com/c/chromium/src/+/974989

Since this commit Autocad has been broken.
We should not let this issue propagate beyond Dev.

Tsuyoshi, are you able to look into this issue?

Ah, yes.
The CL accidentally changed the Service Worker handling logic of synchronous XHR from dedicated worker.
I will create a CL to fix it.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bb9fd1a8249db7ca306513e8d8baf41d3e770674

commit bb9fd1a8249db7ca306513e8d8baf41d3e770674
Author: Tsuyoshi Horo <horo@chromium.org>
Date: Mon Apr 02 03:27:45 2018

Set SkipServiceWorker flag for synchronous loads from the main thread.

Before 5e1b52dd7e828fb2e4bd69f599c0c30eec3e873c, synchronous XHR on worker
was handled by service workers. It is because the |is_sync_load| was false
when the sync request is from worker thread. But after the CL, the
|is_sync_load| flag for the sync request from worker became true, so the request
will not go to the service worker.

This CL will fix this by
 - Set the SkipServiceWorker flag for synchronous loads from the main thread
   in the renderer process. (FetchParameters.cpp)
 - Don't set skip_service_worker even if is_sync_load is true in the browser
   process. (resource_dispatcher_host_impl.cc)

Bug:  706331 , 827473 
Change-Id: I186bc97f3f8d298e0a04942d0ec4b708b3022cc1
Reviewed-on: https://chromium-review.googlesource.com/989376
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547407}
[modify] https://crrev.com/bb9fd1a8249db7ca306513e8d8baf41d3e770674/content/browser/loader/resource_dispatcher_host_impl.cc
[add] https://crrev.com/bb9fd1a8249db7ca306513e8d8baf41d3e770674/third_party/WebKit/LayoutTests/external/wpt/service-workers/service-worker/fetch-request-xhr-sync-on-worker.https.html
[add] https://crrev.com/bb9fd1a8249db7ca306513e8d8baf41d3e770674/third_party/WebKit/LayoutTests/external/wpt/service-workers/service-worker/resources/fetch-request-xhr-sync-on-worker-worker.js
[modify] https://crrev.com/bb9fd1a8249db7ca306513e8d8baf41d3e770674/third_party/WebKit/Source/platform/loader/fetch/FetchParameters.cpp

Thanks for the fast fix!