Epic tree drag and drop: Delay before opening sub epics

Challenge

When dragging and dropping, moving a dragged epic over collapsed epics opens them - this means a lot of extra scrolling.

Proposal

• Hovering on an item (eg epic with many child issues) should not automatically unfurl it. Only when it has been hovered on for the entire duration of the loading period should it unfurl.

• If an item is hovered upon and begins loading, it should stop loading if hovering stops before the item unfurls.

• Collapsed items should not unfurl immediately on hover even if they have already been fetched/expanded/loaded. Hovering should still trigger the loading state for unfurling. Scenario: The Dog epic in the epic tree has 8 child issues. The user has already expanded this Dog epic and that information has already been loaded. If the user closes Dog and then hovers an issue over the collapsed Dog epic to add to it, the Dog epic still shows a loading spinner and slight delay before opening.

• Possible idea to pair up on with UX (@uhlexsis ) in the MR: implement a slight pause over the epic before it's automatically expanded. The first review should be done after completing the above items, and then a delay could perhaps be introduced.