General combinations of tabbing around the MR dialog and then pressing [enter] works fine.

There's a consistent repro for a focus navigation failure in scenarios where...
1. Open MR dialog.
2. Tab to (or use mouse) to enter search/filter mode.
3. Leave search/filter mode. This can be done by either pressing [esc], tabbing to the back button and pressing [enter], or clicking the back button with a mouse.
4. Press [tab]. Observe assertion failure. See below.
5. The tab becomes useless (frozen?). I'm unable to exit the dialog, navigate to another URL, right click, etc.

I haven't been able to repro this issue if the user selects (or presses [enter]) on anything except the search/filter components.

ASSERTION FAILED: !current || !isNonFocusableShadowHost(*current)
../../third_party/WebKit/Source/core/page/FocusController.cpp(590) : blink::Element *blink::(anonymous namespace)::findFocusableElementAcrossFocusScopesForward(blink::(anonymous namespace)::ScopedFocusNavigation &)
1   0x155c35b7f blink::(anonymous namespace)::findFocusableElementAcrossFocusScopesForward(blink::(anonymous namespace)::ScopedFocusNavigation&)
2   0x155c307f0 blink::(anonymous namespace)::findFocusableElementAcrossFocusScopes(blink::WebFocusType, blink::(anonymous namespace)::ScopedFocusNavigation&)
3   0x155c2efea blink::FocusController::advanceFocusInDocumentOrder(blink::LocalFrame*, blink::Element*, blink::WebFocusType, bool, blink::InputDeviceCapabilities*)
4   0x155c2ea1b blink::FocusController::advanceFocus(blink::WebFocusType, bool, blink::InputDeviceCapabilities*)
5   0x156778035 blink::FocusController::advanceFocus(blink::WebFocusType, blink::InputDeviceCapabilities*)
6   0x156775532 blink::EventHandler::defaultTabEventHandler(blink::KeyboardEvent*)
7   0x15677516a blink::EventHandler::defaultKeyboardEventHandler(blink::KeyboardEvent*)
8   0x1564531c3 blink::Node::defaultEventHandler(blink::Event*)
9   0x156c84f26 blink::HTMLElement::defaultEventHandler(blink::Event*)
10  0x1566b04a6 blink::EventDispatcher::dispatchEventPostProcess(void*)
11  0x1566af313 blink::EventDispatcher::dispatch()
12  0x1566ad248 blink::EventDispatchMediator::dispatchEvent(blink::EventDispatcher&) const
13  0x1566ad7f5 blink::EventDispatcher::dispatchEvent(blink::Node&, WTF::RawPtr<blink::EventDispatchMediator>)
14  0x156451efa blink::Node::dispatchEventInternal(WTF::RawPtr<blink::Event>)
15  0x1566f249f blink::EventTarget::dispatchEvent(WTF::RawPtr<blink::Event>)
16  0x156774a6c blink::EventHandler::keyEvent(blink::PlatformKeyboardEvent const&)
17  0x12a381d42 blink::WebViewImpl::handleKeyEvent(blink::WebKeyboardEvent const&)
18  0x12a3829f2 non-virtual thunk to blink::WebViewImpl::handleKeyEvent(blink::WebKeyboardEvent const&)
19  0x12a05a86f blink::PageWidgetDelegate::handleInputEvent(blink::PageWidgetEventHandler&, blink::WebInputEvent const&, blink::LocalFrame*)
20  0x12a37e688 blink::WebViewImpl::handleInputEvent(blink::WebInputEvent const&)
21  0x13123699b content::RenderWidgetInputHandler::HandleInputEvent(blink::WebInputEvent const&, ui::LatencyInfo const&, content::InputEventDispatchType)
22  0x13177bf32 content::RenderWidget::OnHandleInputEvent(blink::WebInputEvent const*, ui::LatencyInfo const&, content::InputEventDispatchType)
23  0x1317ba2e2 void base::DispatchToMethodImpl<content::RenderWidget*, void (content::RenderWidget::*)(blink::WebInputEvent const*, ui::LatencyInfo const&, content::InputEventDispatchType), blink::WebInputEvent const*, ui::LatencyInfo, content::InputEventDispatchType, 0ul, 1ul, 2ul>(content::RenderWidget* const&, void (content::RenderWidget::*)(blink::WebInputEvent const*, ui::LatencyInfo const&, content::InputEventDispatchType), std::__1::tuple<blink::WebInputEvent const*, ui::LatencyInfo, content::InputEventDispatchType> const&, base::IndexSequence<0ul, 1ul, 2ul>)
24  0x1317b9e85 void base::DispatchToMethod<content::RenderWidget*, void (content::RenderWidget::*)(blink::WebInputEvent const*, ui::LatencyInfo const&, content::InputEventDispatchType), blink::WebInputEvent const*, ui::LatencyInfo, content::InputEventDispatchType>(content::RenderWidget* const&, void (content::RenderWidget::*)(blink::WebInputEvent const*, ui::LatencyInfo const&, content::InputEventDispatchType), std::__1::tuple<blink::WebInputEvent const*, ui::LatencyInfo, content::InputEventDispatchType> const&)
25  0x1317b9b5f void IPC::DispatchToMethod<content::RenderWidget, void (content::RenderWidget::*)(blink::WebInputEvent const*, ui::LatencyInfo const&, content::InputEventDispatchType), void, std::__1::tuple<blink::WebInputEvent const*, ui::LatencyInfo, content::InputEventDispatchType> >(content::RenderWidget*, void (content::RenderWidget::*)(blink::WebInputEvent const*, ui::LatencyInfo const&, content::InputEventDispatchType), void*, std::__1::tuple<blink::WebInputEvent const*, ui::LatencyInfo, content::InputEventDispatchType> const&)
26  0x13177bdc5 bool IPC::MessageT<InputMsg_HandleInputEvent_Meta, std::__1::tuple<blink::WebInputEvent const*, ui::LatencyInfo, content::InputEventDispatchType>, void>::Dispatch<content::RenderWidget, content::RenderWidget, void, void (content::RenderWidget::*)(blink::WebInputEvent const*, ui::LatencyInfo const&, content::InputEventDispatchType)>(IPC::Message const*, content::RenderWidget*, content::RenderWidget*, void*, void (content::RenderWidget::*)(blink::WebInputEvent const*, ui::LatencyInfo const&, content::InputEventDispatchType))
27  0x131779943 content::RenderWidget::OnMessageReceived(IPC::Message const&)
28  0x1316cbdac content::RenderViewImpl::OnMessageReceived(IPC::Message const&)
29  0x13eb66a9d IPC::MessageRouter::RouteMessage(IPC::Message const&)
30  0x12fc5527e content::ChildThreadImpl::ChildThreadMessageRouter::RouteMessage(IPC::Message const&)
31  0x13eb6699c IPC::MessageRouter::OnMessageReceived(IPC::Message const&)

btolsch: can you take a look?