(Very drafty for now... just some thoughts).

This Issue is a proposal for how to build into TG v2 a consistent way for a View to have a UI element show itself outside of the View's Viewport.

Popover - A View that that is displayed outside of the View that owns it's Viewport.
 
There are at least these use-cases of Popovers:

Autocomplete Popup

TextView would like a list of autocomplete items to be displayed below the view, at the cursor position as the user types. The user can use mouse/keyboard to select an autocomplete item and the pop over disappears.

Current Implementation

• Adds _popup (a ListView) to this.SuperView
• Uses Visible to show/hide
• (In Partially Adresses #2491. Refactors how Focus works #3627 I've had to change the implementation so _popup only gets removed/disposed when the host is removed/disposed).
   

Drop-down Combobox

Current Implementation

• Does not support pop-over; The ComboBox needs to be sized to allow the drop-down to show if HideDropdownListOnClick is true.
• _listview (of type ComboListView) is a subview of Combobox
• ComboListView overrides OnMouseEvent (which is internal!) with a bunch of hackery to deal with when it should be displayed

Menus (from MenuBar)

Current Implementation

• MenuBar is a "special" subview of Application.Top (Toplevel has knowledge of it).
• When MenuBar is activated (via keyboard/mouse/api) it creates instances of Menu and adds them to Top
• Both MenuBar and Menu have convoluted logic for sensing when the menus should close (via mouse & keyboard).
• When an open Menu closes, it is removed from Top

Context Menus

Current Implementation

• Uses a "fake" MenuBar that is created when Show is called.
• _menuBar is never actually added to any views; BeginInit/EndInit are called manually.

Tooltip

As a user, when I hover the mouse over a View, I'd like a temporary popup to appear providing a "tip" regarding the View's purpose.

Current Implementation

• Not currently implemented

Proposal

Tenets (Unless you know better ones...)

• There can only be one - Just like in Highlander, there can only be one Popover visible and active to the user at a time. None of the use-cases above lead to needing to have more than one, thus this is a simplifier. ¹
• Its Rude to Mess With Someone Elses' Subviews - Asking View developers to have to code defensively around the fact that some Subview they add, or some other agent, is adding/removing subviews is just bad juju. It leads to bugs in object lifecycles and requires skill to get right (e.g. adding a Subview during draw can result in Subviews being changed during a foreach iteration). In this design we will ensure there's no need to mess with another View's Subviews.
• Any View can be a Popover - The design should not require a View that "Pops Over" be coded specially for that purpose.

Design

At the Application level we'll add a peer of Application.Top named Application.Popover:

• In the run loop, after Top.Draw() is called, if Popover.Visible we'll call Popover.Draw(). This will ensure the Popover is always drawn over everything else.
• In keyboard handling, if Popover.Visible && Popover.HasFocus we'll give Popover a chance to handle key events.
• In mouse handling, if Popover.VIsible and Popover == Applicaiton.MouseGrabView we'll let it have mouse events. If a click happens outside of Popover.Frame we set Popover.Visible = false.
• In focus/navigation handling, if Popover.HasFocus, if any other View gets focus, we'll set Popover.Visible = false.
• For layout, whenever Top.LayoutSubviews is called, we'll then call Popover.SetRelativeLayout
• Two APIs:
  • Applicaton.ShowPopover (View popoverView)

public static bool ShowPopover (View popoverView) 
{
   if (Popover is {})
   {
      Popover.Visible = false;
   }

   if (!popoverView.IsInitialized)
   {
      popoverView.BeginInit();
      popoverView.EndInit();
   }

   Popover = popoverView;
   Popover.Visible = true;
   Popover.SetRelativeLayout(screen);
}

• Application.HidePopover ()

public static void HidePopover () 
{
   if (Popover is {})
   {
      Popover.Visible = false;
   }
}

View will have no knowledge of the popover concept.

Any View subclass that wants a Popover, will:

• _myPopover = new MyPopoverView();
• Note, _myPopover will always have this.SuperView is null.
• Any time the popover should be displayed:
  • Set _myPopover.X/Y/Width/Height to ??? (NEED TO FIGURE THIS OUT)
  • Call Application.ShowPopover(_myPopover)
• If the view wants to explicitly hide the popover,
  • Call Application.HidePopover()