Navigation Deep Dive

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?
What are the default actions for standard key/mouse input (e.g. Hotkey, Space, Enter, MouseClick)?

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.

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.

HotKeys

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

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, 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).

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

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.

The FocusedChanged and @Terminal.Gui.ApplicationNavigation.FocusedChanging events are 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.

The AdvanceFocus(NavigationDirection, TabBehavior?) method 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).

AdvanceFocus(NavigationDirection, TabBehavior?) is the primary method for developers to cause a view to gain or lose focus.

Various events are raised when a View's focus is changing. For example, HasFocusChanging and HasFocusChanged.

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.

Knowing When a View's Focus is Changing

HasFocusChanging and HasFocusChanged are raised when a View's focus is changing.

Built-In Views Interactivity

				Keyboard				Mouse
	Number of States	Static	IsDefault	Hotkeys	Select Command	Accept Command	Hotkey Command	CanFocus Click	CanFocus DblCLick	!CanFocus Click	RightClick	GrabMouse
View	1	Yes	No	1	OnSelect	OnAccept	Focus	Focus				No
Label	1	Yes	No	1	OnSelect	OnAccept	FocusNext	Focus		FocusNext		No
Button	1	No	Yes	1	OnSelect	Focus OnAccept	Focus OnAccept	HotKey		Select		No
Checkbox	3	No	No	1	OnSelect Advance	OnAccept	OnAccept	Select		Select		No
RadioGroup	> 1	No	No	2+	Advance	Set SelectedItem OnAccept	Focus Set SelectedItem	SetFocus Set _cursor		SetFocus Set _cursor		No
Slider	> 1	No	No	1	SetFocusedOption	SetFocusedOption OnAccept	Focus	SetFocus SetFocusedOption		SetFocus SetFocusedOption		Yes
ListView	> 1	No	No	1	MarkUnMarkRow	OpenSelectedItem OnAccept	OnAccept	SetMark OnSelectedChanged	OpenSelectedItem OnAccept			No

Accesibilty Tenets

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

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

Table of Contents

Lexicon & Taxonomy

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

Keyboard Navigation

HotKeys

Mouse Navigation

Application Level Navigation

View Level Navigation

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?

Knowing When a View's Focus is Changing

Built-In Views Interactivity

Accesibilty Tenets

Table of Contents

Navigation Deep Dive

Lexicon & Taxonomy

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

Keyboard Navigation

HotKeys

Mouse Navigation

Application Level Navigation

View Level Navigation

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?

Knowing When a View's Focus is Changing

Built-In Views Interactivity

Accesibilty Tenets

How Does `View.Add/Remove` Work?