Add .md doc for Windows Virtual Desktop support

Change-Id: I4f58aa906bc7f8c1f82d06e56099ad6927cb4680
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2406533Reviewed-by: David Bienvenu <davidbienvenu@chromium.org>
Reviewed-by: Jesse McKenna <jessemckenna@google.com>
Commit-Queue: David Bienvenu <davidbienvenu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806350}

docs/windows_virtual_desktop_handling.md

+#  Windows 10 Virtual Desktop support
+
+Windows 10 introduced Virtual Desktop support. Virtual Desktops are similar to
+Chrome OS and Mac workspaces. A virtual desktop is a collection of windows.
+Every window belongs to a virtual desktop.  When a virtual desktop is selected
+to be active, the windows associated with that virtual desktop are displayed on
+the screen. When a virtual desktop is hidden, all of its windows are also
+hidden. This enables the user to create multiple working environments and to
+switch between them. An app (e.g., Chromium) can have windows open on
+different virtual desktops, and thus may need to be Virtual Desktop-aware.
+
+The user-facing Chromium support for virtual desktops consists of two things:
+
+  * When launching the browser with session restore, browser windows are moved
+  to the virtual desktop they were on when the browser shutdown.
+  * When opening a URL with the browser, either open it in a window on the
+  current virtual desktop, or open a new window on the current virtual desktop.
+  Don't open it in a tab in a window on another virtual desktop.
+
+The core UI principles are that windows should be restored to the desktop they
+were shut down on, and opening an app window shouldn't change the current
+virtual desktop. Only the user should be able to change virtual desktops, or
+move windows between virtual desktops.
+
+Windows 10 exposes the COM interface
+[IVirtualDesktopManager](https://docs.microsoft.com/en-us/windows/win32/api/shobjidl_core/nn-shobjidl_core-ivirtualdesktopmanager)
+to access the Virtual Desktop functionality. To make sure that opening a URL
+stays on the current virtual desktop,
+[BrowserView::IsOnCurrentWorkspace](https://source.chromium.org/chromium/chromium/src/+/master:chrome/browser/ui/views/frame/browser_view.cc?q=%20BrowserView::IsOnCurrentWorkspace)
+uses the IVirtualDesktopManager method
+[IsWindowOnCurrentVirtualDesktop](https://docs.microsoft.com/en-us/windows/win32/api/shobjidl_core/nf-shobjidl_core-ivirtualdesktopmanager-iswindowoncurrentvirtualdesktop).
+[BrowserMatches](https://source.chromium.org/chromium/chromium/src/+/master:chrome/browser/ui/browser_finder.cc?q=BrowserMatches)
+in browser_finder.cc only returns a browser window on the current desktop.
+
+To restore browser windows to the desktop they were last open on,
+[BrowserDesktopWindowTreeHostWin](https://source.chromium.org/chromium/chromium/src/+/master:chrome/browser/ui/views/frame/browser_desktop_window_tree_host_win.cc)
+implements GetWorkspace by using the
+[GetWindowDesktopId](https://docs.microsoft.com/en-us/windows/win32/api/shobjidl_core/nf-shobjidl_core-ivirtualdesktopmanager-getwindowdesktopid) method on
+IVirtualDesktopManager, and restores the workspace using
+[MoveWindowToDesktop](https://docs.microsoft.com/en-us/windows/win32/api/shobjidl_core/nf-shobjidl_core-ivirtualdesktopmanager-movewindowtodesktop),
+in its ::Init method.
+
+The actual implementation is a bit more complicated in order to avoid
+calling COM methods on the UI thread, or destroying COM objects on the UI
+thread, since doing so can cause nested message loops and re-entrant calls,
+leading to blocked UI threads and crashes. The
+[VirtualDesktopHelper](https://source.chromium.org/chromium/chromium/src/+/master:chrome/browser/ui/views/frame/browser_desktop_window_tree_host_win.cc?q=VirtualDesktopHelper&sq=&ss=chromium%2Fchromium%2Fsrc)
+class does the workspace handling for BrowserDesktopWindowTreeHostWin, including
+doing all the COM operations on a separate COM task runner. The GetWorkspace
+method is synchronous so VirtualDesktopHelper has to remember and return the
+most recent virtual desktop. Windows has no notification of a window changing
+virtual desktops, so BrowserDesktopWindowTreeHostWin updates the virtual desktop
+of a window whenever it gets focus, and if it has changed, calls
+WindowTreeHost::OnHostWorkspaceChanged. This means that if a window is moved
+to a different virtual desktop, but doesn't get focus before the browser is shut
+down, the browser window will be restored to the previous virtual desktop.
+
+Windows on different virtual desktops share the same coordinate system, so code
+that iterates over windows generally needs to be Virtual Desktop-aware.
+For example, the following places in code ignore windows not on the current
+virtual desktop:
+
+ * [LocalProcessWindowFinder::GetProcessWindowAtPoint](https://source.chromium.org/chromium/chromium/src/+/master:ui/display/win/local_process_window_finder_win.cc?q=LocalProcessWindowFinder::ShouldStopIterating&ss=chromium%2Fchromium%2Fsrc)
+ * third_party/webrtc/modules/desktop_capture/win/window_capture_utils.cc
+ * [Native Window occlusion tracker](https://source.chromium.org/chromium/chromium/src/+/master:ui/aura/native_window_occlusion_tracker_win.cc?q=WindowCanOccludeOtherWindowsOnCurrentVirtualDesktop&ss=chromium%2Fchromium%2Fsrc),
+ when determining if a Chromium window is occluded/covered by other windows.
+ Windows not on the current virtual desktop are considered occluded.
+
+