What happens on a click

chrome_click.cc

// actual differences (in chrome, IE, Firefox, Safari an Edge are all different) are:
// - behavior in disabled 'form' controls
// - phoning to home base (not here, it's defined elsewhere, can dig it up)
// - dealing with clicks dispatching clicks 'recursively'

// this is HTMLElement::click
void HTMLElement::click() {
  DispatchSimulatedClick(nullptr, kSendNoEvents,
                         SimulatedClickCreationScope::kFromScript);
}

// this is the DispatchSimulatedClick method in event_dispatcher.cc the "heart" of the difference
void EventDispatcher::DispatchSimulatedClick(
    Node& node,
    Event* underlying_event,
    SimulatedClickMouseEventOptions mouse_event_options,
    SimulatedClickCreationScope creation_scope) {
  // This persistent vector doesn't cause leaks, because added Nodes are removed
  // before dispatchSimulatedClick() returns. This vector is here just to
  // prevent the code from running into an infinite recursion of
  // dispatchSimulatedClick().
  DEFINE_STATIC_LOCAL(Persistent<HeapHashSet<Member<Node>>>,
                      nodes_dispatching_simulated_clicks,
                      (MakeGarbageCollected<HeapHashSet<Member<Node>>>()));

  if (IsDisabledFormControl(&node))
    return;

  if (nodes_dispatching_simulated_clicks->Contains(&node))
    return;

  nodes_dispatching_simulated_clicks->insert(&node);

  if (mouse_event_options == kSendMouseOverUpDownEvents)
    EventDispatcher(node, *MouseEvent::Create(event_type_names::kMouseover,
                                              node.GetDocument().domWindow(),
                                              underlying_event, creation_scope))
        .Dispatch();

  if (mouse_event_options != kSendNoEvents) {
    EventDispatcher(node, *MouseEvent::Create(event_type_names::kMousedown,
                                              node.GetDocument().domWindow(),
                                              underlying_event, creation_scope))
        .Dispatch();
    node.SetActive(true);
    EventDispatcher(node, *MouseEvent::Create(event_type_names::kMouseup,
                                              node.GetDocument().domWindow(),
                                              underlying_event, creation_scope))
        .Dispatch();
  }
  // Some elements (e.g. the color picker) may set active state to true before
  // calling this method and expect the state to be reset during the call.
  node.SetActive(false);

  // always send click
  EventDispatcher(node, *MouseEvent::Create(event_type_names::kClick,
                                            node.GetDocument().domWindow(),
                                            underlying_event, creation_scope))
      .Dispatch();

  nodes_dispatching_simulated_clicks->erase(&node);
}

And there is a separate path for 'real' clicks as well as clicks through the debugger (Input.dispatchMouseEvent which is essentially what Selenium calls).

This is part of why automation is hard.