Wrong event triggered by ApiController::attachToSlot()

Problem/Motivation

ON_ATTACH_TO_ROOT is unexpected here:

public function attachToSlot(Request $request, string $builder_id, string $instance_id, string $slot): HtmlResponse {
...
  return $this->dispatchDisplayBuilderEvent(
    $is_move ? DisplayBuilderEvents::ON_MOVE : DisplayBuilderEvents::ON_ATTACH_TO_ROOT,
    $builder_id,
    NULL,
    $instance_id,
    $parent_id,
  );

Same onAttachToRoot & onAttachToSlot:

• Buttons: HistoryButtons, StateButtons, ActiveUsers
• Contextual: UiStylesPanel, UiSkinsPanel, InfoPanel, InstanceFormPanel, VisibilityConditionsPanel
• View: LogsPanel, PreviewPanel

Different onAttachToRoot & onAttachToSlot:

• BuilderPanel and its children: LayersPanel, TreePanel

This must affect performance (we are sending way too much HTML than necessary) and this may explain some weird behaviours there.

Proposed resolution

Fix ApiController::attachToSlot() and test BuilderPanel LayersPanel & TreePanel. Maybe some JS or PHP mechanism were built around this error.