titlebar spec initial

specs/TitleBar/titleBar-dev-spec.md

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+<!-- The purpose of this document is to describe the design and implementation of a new WinUI control.
+     This document contains architectural and implementation details that do not appear in the functional spec. -->
+
+<!-- 
+Author: Karen
+Audiences: Github community contributors to learn about the control, provide feedback 
+
+This PR will be open for feedback for a month, 10/11-11/11
+For more info on the public api review process: https://aka.ms/winappsdk/api-specs-review 
+-->
+
+# Description
+
+<!-- Use this section to provide a brief description of the control. -->
+The WinUI TitleBar is a title bar control that allows easy additions of WinUI components such as AutoSuggestBox, PersonPicture, and Mica theming. 
+It is to be used in lieu of the Shell title bar for when developers want more than the basic shell title bar functionalities. 
+
+Current implementations of a custom titlebar involves the developer to fully create a user component from scratch. A notable painpoint in this process is 
+to manually calculate drag regions for when there are elements in the title bar (ie. AutoSuggestBox - refer to WinUI-Gallery sample). WinUI titleBar is meant 
+to encapsulate the most common design scenarios and streamline this process. 
+
+# Design considerations
+
+## AppWindow TitleBar (IXP) vs. WinUI TitleBar
+
+![Basic TitleBar](images/titlebar-basic.png)
+
+### AppWindow.TitleBar (IXP)
+The AppWindow TitleBar is the default title bar, and downlevels to Win32.
+
+AppWindow drawn title bar covers the basic functionalities:
+- AppWindow.SetIcon
+- AppWindow.Title
+- Show Max/Min/Close caption buttons
+- Respects theme
+
+<br>
+
+### WinUI TitleBar
+Developers will 'opt-in' to the WinUI TitleBar for updated styling and extended functionalities.
+
+**Functionalities:**
+- HeaderArea (ie. BackButton, PaneToggleButton)  
+![alt text](images/titlebar-header-icongroup.png)
+
+- Icon  
+![alt text](images/titlebar-icon.png)
+
+- Title  
+![alt text](images/titlebar-title.png)
+
+- Subtitle  
+![alt text](images/titlebar-subtitle.png)
+
+- ContentArea (ie. AutoSuggestBox)  
+![alt text](images/titlebar-search.png)
+
+- FooterArea (ie. PersonPicture)
+![alt text](images/titlebar-personpicture.png)
+
+- Transparency for Mica
+- Custom Height - 32px as default / 48px if content != null  
+![TitleBar Size Variants](images/titlebar-sizevariant.png)
+
+## Considerations
+
+- Punch hole through Header / Content / Footer for drag regions. Calculations needs to be updated on SizeChanged.
+     - Regions with hole punched through are non-draggable.
+
+- Currently TitleBar treats all custom content as interactable and punches a whole through the drag region. 
+How can we allow such customizations? 
+     - If we add an `IsInteractive` attached property, that means TitleBar will have to walk through the 
+     entire visual tree to calculate the rects every time. 
+     - Can we call it `InteractiveContent` instead? But this limits the customizability of the control.
+     - Reference Teams forward / backward button. It is draggable when disabled.
+
+- Interactive content in TitleBar must also be capable of keyboard navigation. 
+They should be treated as a normal elements in the app's visual tree.
+
+- CoreApplicationViewTitleBar.LayoutMetricsChanged and IsVisibleChanged events from the UWP world is 
+missing in WinUI3:
+     - LayoutMetricsChanged: Update Left and Right Inset for on DPI changes. 
+     Right Inset is used to give placeholder space for the caption buttons in WinUI TitleBar.
+     - IsVisibleChanged: This is used for when a Window is in full screen mode, 
+     and WinUI TitleBar needs to know when to collapse itself.
+     - Needs exposed events or dependency property so values can be updated dynamically.
+
+- Minimum drag region for accessibility (ie. Gap between PersonPicture and CaptionButton for Store)  
+
+- TitleBar behaviour with smoke layer with ContentDialog. Should TitleBar still be draggable 
+when content dialog is visible? If so, a disable drag bool API is needed.
+https://github.com/microsoft/microsoft-ui-xaml/issues/6534 for reference.
+
+- Flyout needs to be aware of TitleBar dimensions so it can calculate whether it should expand up / down.
+
+
+<br><br>
+
+![Store TitleBar](images/1p-store-titlebar.png)
+![Store TitleBar MinimalMode](images/1p-store-titlebar-minimal.png)
+
+- AutoSuggestBox to collapse as an icon when in minimal mode.
+     - Argument for having search as part of titleBar
+- MinSize for the apps itself:
+     - Store has set a MinSize which negates the need to fully collapse its AutoSuggestBox
+     - Design does give window sizing guidance, but should test for worst case scenario
+- Which elements collapses on re-size
+     - Store collapses Title and Subtitle in minimal mode
+
+<br><br>
+
+![WinUI-Gallery TitleBar](images/1p-winuigallery-titlebar.png)
+![WinUI-Gallery TitleBar Minimal Mode](images/1p-winuigallery-titlebar-minimal.png)
+- Handling of BackButton and PaneToggleButton for NavView
+     - Collapsing and moving pane toggle button to titlebar in Minimal Mode
+
+<br><br>
+
+![Teams TitleBar](images/1p-teams-titlebar.png)
+- Ability to add non standard elements such as "<", ">" in content, and other buttons in footer
+
+<br><br>
+
+![Visual Studio Code TitleBar](images/1p-vscode-titlebar.png)
+- Ability to toggle Header content before or after TitleBar icon? 
+- _Currently not implemented_
+
+# Implementation Details
+
+## Hole-punching through Non-Client Area
+Once TitleBar control is set as the Window titlebar in developer codebehind, the entire region's input is marked as non-client
+and becomes non-interactable. We need to punch out a "hole" for each interactable region in TitleBar.
+Eg. Header, body, and footer content areas.
+
+`m_interactableElementsList`
+- A list of interactable elements is tracked in the lifetime of the TitleBar.
+- The list is updated everytime the specified elements' visibility changes.
+     - eg. `BackButton`, `PaleToggleButton`, `Header`, `Content`, `Footer`.
+
+`UpdateDragRegion()`
+- Called everytime `SizeChanged` and `LayoutUpdated` events are fired.
+- Checks if `m_interactableElementsList` has children, and if so gets the rects of each element.
+- Accesses the `InputNonClientPointerSource` at the IXP layer and set rects as pass through regions.
+
+_Note: a custom "LayoutUpdated" event needs to be written._
+_The Default LayoutUpdated event fires for every little (non-relevant) events._
+
+
+# Functional spec link
+
+[Functional Spec](images/titleBar-functional-spec.md)
+
+# Architectural overview
+
+## Key public components
+
+TitleBar class
+
+## Key internal components
+
+TitleBar TemplateSettings 
+     - Calculate DragRegions
+
+TitleBarAutomationPeer
+
+## Gesture recognizer handling
+
+TitleBar must handle mouse, touch, and keyboard scenarios.
+
+# Appendix
+
+## Non-considerations
+
+TitleBar is not needed for TabView scenarios - it may be considered as a separate `TabbedTitleBar` control. 
+
+Below scenarios demonstrate that when Tabview occupies the entire width when extending into title bar. 
+Header icons, buttons, and drag regions can be handled with TabView Header, and FooterArea.
+
+
+![Terminal TitleBar](images/1p-terminal-titlebar.png)
+![NotePad TitleBar with TabView](images/1p-notepad-titlebar.png)
+![Edge TitleBar](images/1p-edge-titlebar.png)
+- No backbutton, no other elements, just tabView
+- TabView extends into titlebar takes up the whole window
+- TabView needs a property to make space for icon or other elements 
+- TabView also needs to make space for drag region on right.
+     - FileExplorer currently has their own implement by setting a drag region and CaptionControl placeholder in TabView.FooterArea