What are the visual cues that help the user know which element of an application is receiving keyboard and mouse input (which one has focus)?
How does the user change which element of an application has focus?
How does the user change which element of an application has focus?
What are the visual cues that help the user know what keystrokes will change the focus?
What are the visual cues that help the user know what keystrokes will cause action in elements of the application that don't currently have focus?
What is the order in which UI elements are traversed when using keyboard navigation?

Lexicon & Taxonomy

Navigation refers to the user experience for moving focus between views in the application view-hierarchy.
Focus - Refers to the state where a particular UI element (View), such as a button, input field, or any interactive component, is actively selected and ready to receive user input. When an element has focus, it typically responds to keyboard events and other interactions.
Focus Chain - The ordered sequence of UI elements that can receive focus, starting from the currently focused element and extending to its parent (SuperView) elements up to the root of the focus tree (Application.Top). This chain determines the path that focus traversal follows within the application. Only one focus chain in an application can have focus (top.HasFocus == true), and there is one, and only one, View in a focus chain that is the most-focused; the one receiving keyboard input.
Cursor - A visual indicator to the user where keyboard input will have an impact. There is one Cursor per terminal session. See Cursor for a deep-dive.
Focus Ordering - The order focusable Views are navigated. Focus Ordering is typically used in UI frameworks to enable screen readers and improve the Accessibility of an application. In v1, TabIndex/TabIndexes enabled Focus Ordering.
Tab - Describes the Tab key found on all keyboards, a break in text that is wider than a space, or a UI element that is a stop-point for keyboard navigation. The use of the word "Tab" for this comes from the typewriter, and is reinforced by the existence of a Tab key on all keyboards.
TabStop - A View that is an ultimate stop-point for keyboard navigation. In this usage, ultimate means the View has no focusable subviews. The Application.NextTabStopKey and Application.PrevTabStopKey are Key.Tab and Key.Tab.WithShift respectively. These keys navigate only between peer-views.
TabGroup - A View that is a container for other focusable views. The Application.NextTabGroupKey and Application.PrevTabGroupKey are Key.PageDown.WithCtrl and Key.PageUp.WithCtrl respectively. These keys enable the user to use the keyboard to navigate up and down the view-hierarchy.
Enter / Gain - Means a View that previously was not focused is now becoming focused. "The View is entering focus" is the same as "The View is gaining focus". These terms are legacy terms from v1.
Leave / Lose - Means a View that previously was focused is now becoming un-focused. "The View is leaving focus" is the same as "The View is losing focus". These terms are legacy terms from v1.

See the Keyboard Tenets as they apply as well.

Tenets higher in the list have precedence over tenets lower in the list.

One Focus Per App - It should not be possible to have two views be the "most focused" view in an application.
There's Always a Way With The Keyboard - The framework strives to ensure users' wanting to use the keyboard can't get into a situation where some element of the application is not accessible via the keyboard. For example, we have unit tests that ensure built-in Views will all have at least one navigation key that advances focus. Another example: As long as a View with a HotKey is visible and enabled, regardless of view-hierarchy, if the user presses that hotkey, the action defined by the hotkey will happen (and, by default the View that defines it will be focused).
Flexible Overrides - The framework makes it easy for navigation changes to be made from code and enables changing of behavior to be done in flexible ways. For example a view can be prevented from getting focus by setting CanFocus to false or overriding OnHasFocusChanging and returning true to cancel.
Decouple Concepts - In v1 CanFocus is tightly coupled with HasFocus, TabIndex, TabIndexes, and TabStop and vice-versa. There was a bunch of "magic" logic that automatically attempted to keep these concepts aligned. This resulted in a poorly specified, hard-to-test, and fragile API. In v2 we strive to keep the related navigation concepts decoupled. For example, CanFocus and TabStop are decoupled. A view with CanFocus == true can have TabStop == NoStop and still be focusable with the mouse.

Design

The majority of the Terminal.Gui Navigation system is dedicated to enabling the keyboard to be used to navigate Views.

Terminal.Gui defines these keys for keyboard navigation:

Application.NextTabStopKey (Key.Tab) - Navigates to the next subview that is a TabStop (see below). If there is no next, the first subview that is a TabStop will gain focus.
Application.PrevTabStopKey (Key.Tab.WithShift) - Opposite of Application.NextTabStopKey.
Key.CursorRight - Operates identically to Application.NextTabStopKey.
Key.CursorDown - Operates identically to Application.NextTabStopKey.
Key.CursorLeft - Operates identically to Application.PrevTabStopKey.
Key.CursorUp - Operates identically to Application.PrevTabStopKey.
Application.NextTabGroupKey (Key.F6) - Navigates to the next view in the view-hierarchy that is a TabGroup (see below). If there is no next, the first view that is a TabGroup will gain focus.
Application.PrevTabGroupKey (Key.F6.WithShift) - Opposite of Application.NextTabGroupKey.

F6 was chosen to match Windows

These keys are all registered as KeyBindingScope.Application key bindings by Application. Because application-scoped key bindings have the lowest priority, Views can override the behaviors of these keys (e.g. TextView overrides Key.Tab by default, enabling the user to enter \t into text). The AllViews_AtLeastOneNavKey_Leaves unit test ensures all built-in Views have at least one of the above keys that can advance.

`HotKey`

See also Keyboard where HotKey is covered more deeply...

In v2, HotKeys can be used to navigate across the entire application view-hierarchy. They work independently of Focus. This enables a user to navigate across a complex UI of nested subviews if needed (even in overlapped scenarios). An example use case is the AllViewsTester scenario.

Additionally, in v2, multiple Views in an application (even within the same SuperView) can have the same HotKey. Each press of the HotKey will invoke the next HotKey across the View hierarchy (NOT IMPLEMENTED YET see https://github.com/gui-cs/Terminal.Gui/issues/3554).

Mouse-based navigation is straightforward in comparison to keyboard: If a view is focusable and the user clicks on it, it gains focus. There are some nuances, though:

If a View is focusable, and it has focusable sub-views, what happens when a user clicks on the Border of the View? Which sub-view (if any) will also get focus?
If a View is focusable, and it has focusable sub-views, what happens when a user clicks on the ContentArea of the View? Which sub-view (if any) will also get focus?

The answer to both questions is:

If the View was previously focused, the system keeps a record of the Subview that was previously most-focused and restores focus to that Subview (RestoreFocus()).

If the View was not previously focused, AdvanceFocus() is called.

For this to work properly, there must be logic that removes the focus-cache used by RestoreFocus() if something changes that makes the previously-focusable view not focusable (e.g. if Visible has changed).

`Application`

At the application level, navigation is encapsulated within the ApplicationNavigation helper class which is publicly exposed via the Application.Navigation property.

`Application.Navigation.GetFocused ()`

Gets the most-focused View in the application. Will return null if there is no view with focus (an extremely rare situation). This replaces View.MostFocused in v1.

`Application.Navigation.FocusedChanged` and `Application.Navigation.FocusedChanging`

Events raised when the most-focused View in the application is changing or has changed. FocusedChanged is useful for apps that want to do something with the most-focused view (e.g. see AdornmentsEditor). FocusChanging is useful apps that want to override what view can be focused across an entire app.

`Application.Navigation.AdvanceFocus (NavigationDirection direction, TabBehavior? behavior)`

Causes the focus to advance (forward or backwards) to the next View in the application view-hierarchy, using behavior as a filter.

The implementation is simple:

return Application.Current?.AdvanceFocus (direction, behavior);

This method is called from the Command handlers bound to the application-scoped keybindings created during Application.Init. It is public as a convenience.

This method replaces about a dozen functions in v1 (scattered across Application and Toplevel).

`View`

At the View-level, navigation is encapsulated within View.Navigation.cs.

What makes a View focusable?

First, only Views that are visible and enabled can gain focus. Both Visible and Enabled must be true for a view to be focusable.

For visible and enabled Views, the CanFocus property is then used to determine whether the View is focusable. CanFocus must be true for a View to gain focus. However, even if CanFocus is true, other factors can prevent the view from gaining focus...

A visible, enabled, and CanFocus == true view can be focused if the user uses the mouse to clicks on it or if code explicitly calls View.SetFocus(). Of course, the view itself or some other code can cancel the focus (e.g. by overriding OnEnter).

For keyboard navigation, the TabStop property is a filter for which views are focusable from the current most-focused. TabStop has no impact on mouse navigation. TabStop is of type TabBehavior.

null - This View is still being initialized; acts as a signal to set_CanFocus to set TabStop to TabBehavior.TabStop as convince for the most common use-case. Equivalent to TabBehavior.NoStop when determining if a view is focusable by the keyboard or not.
TabBehavior.NoStop - Prevents the user from using keyboard navigation to cause view (and by definition it's subviews) to gain focus. Note: The view can still be focused using code or the mouse.
TabBehavior.TabStop - Indicates a View is a focusable view with no focusable subviews. Application.Next/PrevTabStopKey will advance ONLY through the peer-Views (SuperView.Subviews).
TabBehavior.GroupStop - Indicates a View is a focusable container for other focusable views and enables keyboard navigation across these containers. This applies to both tiled and overlapped views. For example, FrameView is a simple view designed to be a visible container of other views tiled scenarios. It has TabStop set to TabBehavior.GroupStop (and Arrangement set to ViewArrangement.Fixed). Likewise, Window is a simple view designed to be a visible container of other views in overlapped scenarios. It has TabStop set to TabBehavior.GroupStop (and Arrangement set to ViewArrangement.Movable | ViewArrangement.Resizable | ViewArrangement.Overlapped). Application.Next/PrevGroupStopKey will advance across all GroupStop views in the application (unless blocked by a NoStop SuperView).

How To Tell if a View has focus? And which view is the most-focused?

View.HasFocus indicates whether the View is focused or not. It is the definitive signal. If the view has no focusable Subviews then this property also indicates the view is the most-focused view in the application.

Setting this property to true has the same effect as calling View.SetFocus (), which also means the focus may not change as a result.

If v.HasFocus == true then

All views up v's superview-hierarchy must be focusable.
All views up v's superview-hierarchy will also have HasFocus == true.
The deepest-subview of v that is focusable will also have HasFocus == true

In other words, v.HasFocus == true does not necessarily mean v is the most-focused view, receiving input. If it has focusable sub-views, one of those (or a further subview) will be the most-focused (Application.Navigation.Focused).

The private bool _hasFocus field backs HasFocus and is the ultimate source of truth whether a View has focus or not.

How does a user tell?

In short: ColorScheme.Focused.

(More needed for HasFocus SuperViews. The current ColorScheme design is such that this is awkward. See Issue #2381)

How to make a View become focused?

The primary public method for developers to cause a view to get focus is View.SetFocus().

Unlike v1, in v2, this method can return false if the focus change doesn't happen (e.g. because the view wasn't focusable, or the focus change was cancelled).

How to make a View become NOT focused?

The typical method to make a view lose focus is to have another View gain focus.

Determining the Most Focused SubView

In v1 View had MostFocused property that traversed up the view-hierarchy returning the last view found with HasFocus == true. In v2, Application.Focused provides the same functionality with less overhead.

How Does `View.Add/Remove` Work?

In v1, calling super.Add (view) where view.CanFocus == true caused all views up the hierarchy (all SuperViews) to get CanFocus set to true as well.

Also, in v1, if view.CanFocus == true, Add would automatically set TabStop.

In v2, developers need to explicitly set CanFocus for any view in the view-hierarchy where focus is desired. This simplifies the implementation significantly and removes confusing behavior.

In v2, the automatic setting of TabStop in Add is retained because it is not overly complex to do so and is a nice convenience for developers to not have to set both Tabstop and CanFocus. Note we do NOT automatically change CanFocus if TabStop is changed.

Overriding `HasFocus` changes - `OnEnter/OnLeave` and `Enter/Leave`

These virtual methods and events are raised when a View's HasFocus property is changing. In v1 they were poorly defined and weakly implemented. For example, OnEnter was public virtual OnEnter and it raised Enter. This meant overrides needed to know that the base raised the event and remember to call base. Poor API design.

FocusChangingEventArgs.Handled in v1 was documented as

    /// <summary>
    ///     Indicates if the current focus event has already been processed and the driver should stop notifying any other
    ///     event subscriber. It's important to set this value to true specially when updating any View's layout from inside the
    ///     subscriber method.
    /// </summary>

This is clearly copy/paste documentation from keyboard code and describes incorrect behavior. In practice this is not what the implementation does. Instead the system never even checks the return value of OnEnter and OnLeave.

Additionally, in v1 private void SetHasFocus (bool newHasFocus, View view, bool force = false) is confused too complex.

In v2, SetHasFocus () is replaced by private bool EnterFocus (View view) and private bool LeaveFocus (View view). These methods follow the standard virtual/event pattern:

Check pre-conditions:
- For EnterFocus - If the view is not focusable (not visible, not enabled, or CanFocus == false) returns true indicating the change was cancelled.
- For EnterFocus - If CanFocus == true but the SuperView.CanFocus == false throws an invalid operation exception.
- For EnterFocus - If HasFocus is already true throws an invalid operation exception.
- For LeaveFocus - If HasFocus is already false throws an invalid operation exception.
Call the protected virtual bool OnEnter/OnLeave (View?) method. If the return value is true stop and return true, preventing the focus change. The base implementations of these simply return false.
Otherwise, raise the cancelable event (Enter/Leave). If args.Cancel == true stop and return true, preventing the focus change.
Check post-conditions: If HasFocus has not changed, throw an invalid operation exception.
Return false indicating the change was not cancelled (or invalid).

The Enter and Leave events use FocusChangingEventArgs which provides both the old and new Views. FocusChangingEventArgs.Handled changes to Cancel to be more clear on intent.

These could also be named Gain/Lose. They could also be combined into a single method/event: HasFocusChanging.

QUESTION: Should we retain the same names as in v1 to simplify porting? Or, given the semantics of Handled v. Cancel are reversed would it be better to rename and/or combine?

`TabIndex` and `TabIndexes`

v1 Behavior

In v1, within a set of focusable subviews that are TabStops, and within a view hierarchy containing TabGroups, the default order in which views gain focus is the same as the order the related views were added to the SuperView. As superView.Add (view) is called, each view is added to the end of the TabIndexes list.

TabIndex allows this order to be changed without changing the order in SubViews. When view.TabIndex is set, the TabIndexes list is re-ordered such that view is placed in the list after the peer-view with TabIndex-1 and before the peer-view with TabIndex+1.

QUESTION: With this design, devs are required to ensure TabIndex is unique. It also means that set_TabIndex almost always will change the passed value. E.g. this code will almost always assert:

view.TabIndex = n;
Debug.Assert (view.TabIndex == n);

This is horrible API design.

Proposed New Design

In Win32 there is no concept of tab order beyond the Z-order (the equivalent to the order superview.Add was called).

In WinForms the Control.TabIndex property:

can consist of any valid integer greater than or equal to zero, lower numbers being earlier in the tab order. If more than one control on the same parent control has the same tab index, the z-order of the controls determines the order to cycle through the controls.

In WPF the UserControl.Tabindex property:

When no value is specified, the default value is MaxValue. The system then attempts a tab order based on the declaration order in the XAML or child collections.

Terminal.Gui v2 should adopt the WinForms model.

Implementation Plan

A bunch of the above is the proposed design. Eventually Toplevel will be deleted. Before that happens, the implementation will retain dual code paths:

The old Toplevel and OverlappedTop code. Only utilized when IsOverlappedContainer == true
The new code path that treats all Views the same but relies on the appropriate combination of TabBehavior and ViewArrangement settings as well as IRunnable.

Rough Design Notes

Accesibilty Tenets

See https://devblogs.microsoft.com/dotnet/the-journey-to-accessible-apps-keyboard-accessible/

https://github.com/dotnet/maui/issues/1646

Focus Chain & DOM ideas

The navigation/focus code in View.Navigation.cs has been rewritten in v2 (in https://github.com/gui-cs/Terminal.Gui/pull/3627) to simplify and make more robust.

The design is fundamentally the same as in v1: The logic for tracking and updating the focus chain is based on recursion up and down the View.Subviews/View.SuperView hierarchy. In this model, there is the need for tracking state during recursion, leading to APIs like the following:

// From v1/early v2: Note the `force` param.
private void SetHasFocus (bool newHasFocus, View view, bool force = false)

// From #3627: Note the `traversingUp` param
 private bool EnterFocus ([CanBeNull] View leavingView, bool traversingUp = false)

The need for these "special-case trackers" is clear evidence of poor architecture. Both implementations work, and the #3627 version is far cleaner, but a better design could result in further simplification.

For example, moving to a model where Application is responsible for tracking and updating the focus chain instead View. We would introduce a formalization of the Focus Chain.

Focus Chain: A sequence or hierarchy of UI elements (Views) that determines the order in which keyboard focus is navigated within an application. This chain represents the potential paths that focus can take, ensuring that each element can be reached through keyboard navigation. Instead of using recursion, the Focus Chain can be implemented using lists or trees to maintain and update the focus state efficiently at the Application level.

By using lists or trees, you can manage the focus state without the need for recursive traversal, making the navigation model more scalable and easier to maintain. This approach allows you to explicitly define the order and structure of focusable elements, providing greater control over the navigation flow.

Now, the interesting thing about this, is it really starts to look like a DOM!

Designing a DOM (Document Object Model) for UI library involves creating a structured representation of the UI elements and their relationships.

Hierarchy and Structure- Root Node: The top-level node representing the entire application or window.
- View Nodes: Each UI element (View) is a node in the DOM. These nodes can have child nodes, representing nested or contained elements.
Node Properties- Attributes: Each node can have attributes such as id, class, style, and custom properties specific to the View.
- State: Nodes can maintain state information, such as whether they are focused, visible, enabled, etc.
Traversal Methods- Parent-Child Relationships: Nodes maintain references to their parent and children, allowing traversal up and down the hierarchy.
- Sibling Relationships: Nodes can also maintain references to their previous and next siblings for easier navigation.
Event Handling- Event Listeners: Nodes can have event listeners attached to handle user interactions like clicks, key presses, and focus changes.
- Event Propagation: Events can propagate through the DOM, allowing for capturing and bubbling phases similar to web DOM events.
Focus Management- Focus Chain: Maintain a list or tree of focusable nodes to manage keyboard navigation efficiently.
- Focus Methods: Methods to programmatically set and get focus, ensuring the correct element is focused based on user actions or application logic.
Mouse Events - Mouse handling in Terminal.Gui involves capturing and responding to mouse events such as clicks, drags, and scrolls. In v2, mouse events are managed at the View level, but for a DOM-like structure, this should be centralized.
Layout - The Pos/Dim system in Terminal.Gui is used for defining the layout of views. It allows for dynamic positioning and sizing based on various constraints. For a DOM-model we'd maintain the Pos/Dim system but ensure the layout calculations are managed by the DOM manager.
Drawing - Drawing in Terminal.Gui involves rendering text, colors, and shapes. This is handled within the View class today. In a DOM model we'd centralize the drawing logic in the DOM manager to ensure consistent rendering.

This is all well and good, however we are NOT going to fully transition to a DOM in v2. But we may start with Focus/Navigation (item 3 above). Would could retain the existing external View API for focus (e.g. View.SetFocus, Focused, CanFocus, TabIndexes, etc...) but refactor the implementation of those to leverage a FocusChain (or FocusManager) at the Application level.

(Crap code generated by Copilot; but gets the idea across):

public class FocusChain {
    private List<View> focusableViews = new List<View>();
    private View currentFocusedView;

    public void RegisterView(View view) {
        if (view.CanFocus) {
            focusableViews.Add(view);
            focusableViews = focusableViews.OrderBy(v => v.TabIndex).ToList();
        }
    }

    public void UnregisterView(View view) {
        focusableViews.Remove(view);
    }

    public void SetFocus(View view) {
        if (focusableViews.Contains(view)) {
            currentFocusedView?.LeaveFocus();
            currentFocusedView = view;
            currentFocusedView.EnterFocus();
        }
    }

    public View GetFocusedView() {
        return currentFocusedView;
    }

    public void MoveFocusNext() {
        if (focusableViews.Count == 0) return;
        int currentIndex = focusableViews.IndexOf(currentFocusedView);
        int nextIndex = (currentIndex + 1) % focusableViews.Count;
        SetFocus(focusableViews[nextIndex]);
    }

    public void MoveFocusPrevious() {
        if (focusableViews.Count == 0) return;
        int currentIndex = focusableViews.IndexOf(currentFocusedView);
        int previousIndex = (currentIndex - 1 + focusableViews.Count) % focusableViews.Count;
        SetFocus(focusableViews[previousIndex]);
    }
}

Table of Contents

Navigation Deep Dive

Lexicon & Taxonomy

Tenets for Terminal.Gui UI Navigation (Unless you know better ones...)

Design

Keyboard Navigation

`HotKey`

Mouse Navigation

`Application`

`Application.Navigation.GetFocused ()`

`Application.Navigation.FocusedChanged` and `Application.Navigation.FocusedChanging`

`Application.Navigation.AdvanceFocus (NavigationDirection direction, TabBehavior? behavior)`

`View`

What makes a View focusable?

How To Tell if a View has focus? And which view is the most-focused?

How does a user tell?

How to make a View become focused?

How to make a View become NOT focused?

Determining the Most Focused SubView

How Does `View.Add/Remove` Work?

Overriding `HasFocus` changes - `OnEnter/OnLeave` and `Enter/Leave`

`TabIndex` and `TabIndexes`

v1 Behavior

Proposed New Design

Implementation Plan

Rough Design Notes

Accesibilty Tenets

Focus Chain & DOM ideas

Table of Contents

Navigation Deep Dive

Lexicon & Taxonomy

Tenets for Terminal.Gui UI Navigation (Unless you know better ones...)

Design

Keyboard Navigation

HotKey

Mouse Navigation

Application

Application.Navigation.GetFocused ()

Application.Navigation.FocusedChanged and Application.Navigation.FocusedChanging

Application.Navigation.AdvanceFocus (NavigationDirection direction, TabBehavior? behavior)

View

What makes a View focusable?

How To Tell if a View has focus? And which view is the most-focused?

How does a user tell?

How to make a View become focused?

How to make a View become NOT focused?

Determining the Most Focused SubView

How Does View.Add/Remove Work?

Overriding HasFocus changes - OnEnter/OnLeave and Enter/Leave

TabIndex and TabIndexes

v1 Behavior

Proposed New Design

Implementation Plan

Rough Design Notes

Accesibilty Tenets

Focus Chain & DOM ideas

`HotKey`

`Application`

`Application.Navigation.GetFocused ()`

`Application.Navigation.FocusedChanged` and `Application.Navigation.FocusedChanging`

`Application.Navigation.AdvanceFocus (NavigationDirection direction, TabBehavior? behavior)`

`View`

How Does `View.Add/Remove` Work?

Overriding `HasFocus` changes - `OnEnter/OnLeave` and `Enter/Leave`

`TabIndex` and `TabIndexes`