Fixes #2404. Add OpenMenu() methods to MenuBar for dropdown list usage

[Copilot]

Fixes

• Fixes Enable using MenuBar as an alternative for ComboBox #2404

Proposed Changes/Todos

• Add OpenMenu() method to programmatically open first menu item
• Add OpenMenu(Point?) overload for custom positioning
• Update MenuBar documentation with dropdown usage pattern
• Add comprehensive unit tests (3 new tests, all passing)

Changes

Public API Additions

MenuBar.OpenMenu()

• Opens first MenuBarItem with PopoverMenu
• Activates MenuBar and sets focus automatically
• Returns bool indicating success

MenuBar.OpenMenu(Point? position)

• Opens menu at specified screen coordinates
• Enables alignment with adjacent controls (e.g., TextField)
• Falls back to default positioning if null

Implementation

Modified ShowItem() to accept optional Point? position parameter. When provided, positions PopoverMenu at specified coordinates instead of default MenuBarItem bottom-left.

Documentation

Added class-level example showing TextField + MenuBar dropdown pattern:

var tf = new TextField { Width = 10 };
var menuBarItem = new MenuBarItem ("▼", 
    new MenuItem [] {
        new ("Item 1", () => tf.Text = "Item 1"),
        new ("Item 2", () => tf.Text = "Item 2")
    });
var mb = new MenuBar ([menuBarItem]) {
    Width = 1,
    Y = Pos.Top (tf),
    X = Pos.Right (tf)
};
mb.Enter += (s, e) => mb.OpenMenu (new Point (tf.FrameToScreen ().X, tf.FrameToScreen ().Bottom));

Notes

• First menu item selection already works via Menu.OnVisibleChanged - no fix needed
• MenuBar.DefaultBorderStyle already LineStyle.None - clean appearance by default
• All changes additive - no breaking changes

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Enable using MenuBar as an alternative for ComboBox</issue_title>
<issue_description>See comments below.

Essence is the current ComboBox is a bugfarm and barely works. It needs to be replaced.

This Issue is to create a new View named DropDownList.

This comment illustrates how it can be built

#2404 (comment)

</issue_description>

Comments on the Issue (you are @copilot in this section)

@tig Would it make sense to rename this Issue to "Enable using MenuBar as a replacement for ComboBox?" @tig I agree we should have both.

I don't believe "combo" as a term makes sense for this one tho.

Is it really a new view or is it just a MenuBar used in a particular way?</comment_new>
<comment_new>@tig
Do the api docs describe how to do this?</comment_new>
<comment_new>@tig
Why isn't Onfocus true by default?

Yesterday I was goofing with Focus stuff and realized it might make sense for Menubar to participate in tabstops along with the other subviews of a view.</comment_new>
<comment_new>@tig

That's not the normal behavior on a app, but you can do that behavior for Terminal.Gui. It's debatable.

Yeah. Probably best to leave it as is. It was just an idle thought.</comment_new>
<comment_new>@tig
Here's a tweaked scenario. I realy love this!

		public override void Setup ()
		{
			var tf = new TextField () { Width = 10 };
			Application.Top.Add (tf);

			MenuBarItem menu = null;

			MenuItem CreateMenuItem (int i)
			{
				return new MenuItem ($"Item {i}", null, () => {
					tf.Text = menu.Children [i - 1].Title;
				});
			}

			menu = new MenuBarItem ($"{Application.Driver.DownArrow}",
				Enumerable.Range (1, 5).Select (
						(a) => CreateMenuItem (a))
					.ToArray ());

			var mb = new MenuBar (new [] { menu }) {
				CanFocus = true,
				Width = 1,
				Y = Pos.Top (tf),
				X = Pos.Right (tf)
			};

			// HACKS required to make this work:
			mb.Enter += (s, e) => {
				// BUG: This does not select menu item 0
				// Instead what happens is the first keystroke the user presses
				// gets swallowed and focus is moved to 0.  Result is that you have
				// to press down arrow twice to select first menu item and/or have to
				// press Tab twice to move focus back to TextField
				mb.OpenMenu ();
			};

			Application.Top.Add (mb);

This mostly works today as @tznind notes.

What needs to be fixed to make this really be done:

• The menu should pop down aligned to the left of the textField.
• MenuBorderStyle = LineStyle.None might look better

• Fixes Enable using MenuBar as an alternative for ComboBox #2404

---

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.18%. Comparing base (e199063) to head (27023aa).

Additional details and impacted files

@@              Coverage Diff               @@
##           v2_develop    #4412      +/-   ##
==============================================
+ Coverage       77.16%   77.18%   +0.02%     
==============================================
  Files             387      387              
  Lines           44599    44609      +10     
  Branches         6310     6312       +2     
==============================================
+ Hits            34414    34432      +18     
+ Misses           8294     8288       -6     
+ Partials         1891     1889       -2     

Files with missing lines	Coverage Δ
Terminal.Gui/Views/Menu/MenuBar.cs	82.37% <100.00%> (+0.35%)	⬆️

... and 2 files with indirect coverage changes

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