CL:
https://chromium.googlesource.com/chromium/src/+log/58.0.3020.0..58.0.3021.0?pretty=fuller&n=10000
(Unable to narrow down the range via tool bisect since not able to sign in chromium builds)

Suspecting: 452073 ?
Kindly help to reassign if this is not related to your change.

CC+ owners of views/constrained_window . Moving this to P2.

The dialog is presented as a modal dialog for the entire browser window - see https://cs.chromium.org/chromium/src/chrome/browser/ui/views/profiles/signin_view_controller_delegate_views.cc?rcl=f88b1b47594ebba08774469a5f236055104c0381&l=128

I expect this is a general problem with the browser modal dialogs. Re-assigning this to msw@ (owner of views/onstrained_window).
msw@: Please re-assign to me if you think that the sign-in code does something wrong when presenting the sync confirmation dialog.

Sorry for the delay. It seems like there are two separate defects:
A) The tab strip accepts drag-and-drop while a modal dialog is open.
B) Focus can be transferred to the underlying window/page while a modal dialog is open.

I can repro (A) on Linux & ChromeOS with the Views-based Edit Bookmark dialog:
(1) Trigger the Bookmark bubble (CTRL+D)
(2) Click "Edit" to open the Edit Bookmark dialog
(3) Drag text from the "Name" or "URL" textfield into the tab strip.
Expected: The modal dialog prevents drag and drop onto the tab strip.
Actual: The tab strip allows drag and drop while a modal window is open.

I can't actually repro (B) on Linux 61.0.3163.79, does it still repro on Windows?
Defect (B) seems even more severe than (A), if it still repros.

Sadly, I'm not actively working in this area, this issue needs a new owner.

This issue has been Available for over a year. If it's no longer important or seems unlikely to be fixed, please consider closing it out. If it is important, please re-triage the issue.

Sorry for the inconvenience if the bug really should have been left as Available.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot