After https://chromium-review.googlesource.com/c/chromium/src/+/1351805 lands, enabling ToolbarViewInteractiveUITest.TestAppMenuOpensOnDrag on Mac will hang in the run loop that waits for drag and drop to finish:

BrowserTestBase received signal: Terminated: 15. Backtrace:
0   interactive_ui_tests                0x0000000111ab74df base::debug::StackTrace::StackTrace(unsigned long) + 31
1   interactive_ui_tests                0x00000001110c8a25 content::(anonymous namespace)::DumpStackTraceSignalHandler(int) + 229
2   libsystem_platform.dylib            0x00007fff58babf5a _sigtramp + 26
3   ???                                 0x0000000800002e88 0x0 + 34359750280
4   CoreFoundation                      0x00007fff309b1ff5 __CFRunLoopServiceMachPort + 341
5   CoreFoundation                      0x00007fff309b1347 __CFRunLoopRun + 1783
6   CoreFoundation                      0x00007fff309b09b7 CFRunLoopRunSpecific + 487
7   HIToolbox                           0x00007fff2fc90d96 RunCurrentEventLoopInMode + 286
8   HIToolbox                           0x00007fff2fc90b06 ReceiveNextEventCommon + 613
9   HIToolbox                           0x00007fff2fc90884 _BlockUntilNextEventMatchingListInModeWithFilter + 64
10  AppKit                              0x00007fff2df41a73 _DPSNextEvent + 2085
11  AppKit                              0x00007fff2e6d7e34 -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 3044
12  interactive_ui_tests                0x00000001108ed260 __71-[BrowserCrApplication nextEventMatchingMask:untilDate:inMode:dequeue:]_block_invoke + 64
13  interactive_ui_tests                0x00000001119b96ca base::mac::CallWithEHFrame(void () block_pointer) + 10
14  interactive_ui_tests                0x00000001108ed194 -[BrowserCrApplication nextEventMatchingMask:untilDate:inMode:dequeue:] + 164
15  AppKit                              0x00007fff2df36885 -[NSApplication run] + 764
16  interactive_ui_tests                0x00000001119d462c base::MessagePumpNSApplication::DoRun(base::MessagePump::Delegate*) + 364
17  interactive_ui_tests                0x00000001119d28de base::MessagePumpCFRunLoopBase::Run(base::MessagePump::Delegate*) + 110
18  interactive_ui_tests                0x00000001119d0a28 base::MessageLoopImpl::Run(bool) + 136
19  interactive_ui_tests                0x0000000111a0ba89 base::RunLoop::Run() + 249
20  interactive_ui_tests                0x000000010c783477 ToolbarViewInteractiveUITest::DoDragAndDrop(gfx::Point const&, gfx::Point const&) + 663
21  interactive_ui_tests                0x000000010c783de2 ToolbarViewInteractiveUITest_TestAppMenuOpensOnDrag_Test::RunTestOnMainThread() + 642
22  interactive_ui_tests                0x00000001110c7ed8 content::BrowserTestBase::ProxyRunTestOnMainThreadLoop() + 584
23  interactive_ui_tests                0x00000001108f257e ChromeBrowserMainParts::PreMainMessageLoopRunImpl() + 4814
24  interactive_ui_tests                0x00000001108f11ed ChromeBrowserMainParts::PreMainMessageLoopRun() + 45
25  interactive_ui_tests                0x000000010dd43e12 content::BrowserMainLoop::PreMainMessageLoopRun() + 50
26  interactive_ui_tests                0x000000010e33d0f5 content::StartupTaskRunner::RunAllTasksNow() + 117
27  interactive_ui_tests                0x000000010dd428a5 content::BrowserMainLoop::CreateStartupTasks() + 885
28  interactive_ui_tests                0x000000010dd466e9 content::BrowserMainRunnerImpl::Initialize(content::MainFunctionParams const&) + 89
29  interactive_ui_tests                0x000000010dd408bb content::BrowserMain(content::MainFunctionParams const&) + 203
30  interactive_ui_tests                0x000000011044302b content::ContentMainRunnerImpl::RunServiceManager(content::MainFunctionParams&, bool) + 267
31  interactive_ui_tests                0x0000000110442ef0 content::ContentMainRunnerImpl::Run(bool) + 464
32  interactive_ui_tests                0x000000011404067e service_manager::Main(service_manager::MainParams const&) + 3166
33  interactive_ui_tests                0x0000000110441fe4 content::ContentMain(content::ContentMainParams const&) + 68
34  interactive_ui_tests                0x00000001110c7a99 content::BrowserTestBase::SetUp() + 3609
35  interactive_ui_tests                0x000000011087c6de InProcessBrowserTest::SetUp() + 606
36  interactive_ui_tests                0x000000010c7fec90 testing::Test::Run() + 112
37  interactive_ui_tests                0x000000010c7ffbdf testing::TestInfo::Run() + 463
38  interactive_ui_tests                0x000000010c800227 testing::TestCase::Run() + 247
39  interactive_ui_tests                0x000000010c80d7f7 testing::internal::UnitTestImpl::RunAllTests() + 855
40  interactive_ui_tests                0x000000010c80d47d testing::UnitTest::Run() + 109
41  interactive_ui_tests                0x000000011197cdf6 base::TestSuite::Run() + 166
42  interactive_ui_tests                0x000000010c720ac3 InteractiveUITestSuiteRunner::RunTestSuite(int, char**) + 51
43  interactive_ui_tests                0x00000001110e349e content::LaunchTests(content::TestLauncherDelegate*, unsigned long, int, char**) + 622
44  interactive_ui_tests                0x000000011087198e LaunchChromeTests(unsigned long, content::TestLauncherDelegate*, int, char**) + 494
45  interactive_ui_tests                0x000000010c720a2c main + 92
46  libdyld.dylib                       0x00007fff5889d015 start + 1
47  ???                                 0x000000000000000b 0x0 + 11

I don't know why this is.  It happens whether the mouse move that should drag over the app menu button is sent from the UI thread or a background thread.