The reason is:
 - window_modality_controller.cc assumes web modals are transient children of their parent.
 - But, NativeWidgetAura::InitNativeWidget() doesn't add web modals as transient child, because params.child is true for them.
 - params.child is set to true in dialog_delegate.cc:

  // Web-modal (ui::MODAL_TYPE_CHILD) dialogs with parents are marked as child
  // widgets to prevent top-level window behavior (independent movement, etc).
  params.child = parent && (delegate->GetModalType() == ui::MODAL_TYPE_CHILD);

So when we do the naive thing and not key setting params.child on MODAL_TYPE_CHILD (per #1), we break a whole bunch of tests.

For example, PrintPreviewDialogControllerUnitTest.GetOrCreatePreviewDialog. I'm able to bring up the dialog just fine, but it is crashing in the unit tests. This may have something to do with not having a root window?

It seems like purely a problem with tests.

NativeWidgetAura::InitNativeWidget() tries to get the root window when doing anything other than simple direct child attachment. At least all tests that invoke DialogTestBrowserWindow don't have a RootWindow; comments in the constructor calls out that it's tricky to get one.

So fixing the actual bug here is a one liner, but fixing the tests to work afterwards...

The problem runs deeper than this.

The code in window_modality_controller.cc relies on walking across transient children. So the first attempt at fixing is to just ensure that params.child isn't set when building a web modal dialog in DialogDelegate. Some tests break, but if I go ahead and fix the tests by threading a GetContext() to the dialog, all the tests pass. Great. But there are no tests for dialog location, and in all chromeos, these dialogs are mispositioned. It looks like a location/root_location problem again.

This isn't mus specific, and has been broken without comment for almost a year and a half. It's also really difficult to fix due to a bunch of assumptions in the dialog handling code.

If anyone wants to take this, go ahead. This sits at the bottom of my queue.

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