When Chrome is installed on Windows, it creates a shortcut on the desktop that launches Chrome. It also adds the same shortcut to the start menu. These shortcuts do not specify a profile, so they launch Chrome with the most recently used profile.
Windows allows users to pin applications to the taskbar. When a user pins an application to the taskbar, Windows looks for a desktop shortcut that matches the application, and if it finds one, it creates a .lnk file in the directory <user dir>\AppData\Roaming\Microsoft\Internet Explorer\Quick Launch\User Pinned\TaskBar. If it does not find a matching desktop shortcut, it creates an 8-hex-digit sub-directory of <user dir>\AppData\Roaming\Microsoft\Internet Explorer\Quick Launch\ImplicitAppShortcuts\ and puts the .lnk file in that directory. For example, 3ffff1b1b170b31e.
App windows on Windows have an App User Model ID (AUMI) property. For Chrome windows, this is set in BrowserWindowPropertyManager::UpdateWindowProperties, when a window is opened. Windows desktop shortcuts have an app model property, and this should match the open window's AUMI. Windows groups open windows with the same AUMI to a taskbar icon.
There are two kinds of Chrome windows with AUMI's: browser windows, and app windows, which include web apps, and extensions, i.e., windows opened via --app-id or --app.
GetAppUserModelIdForBrowser constructs an AUMI for a browser window and GetAppUserModelIdForApp constructs an AUMI for an app window. Each calls ShellUtil::BuildAppUserModelId to construct the AUMI out of component strings.
All AUMI's start with the base app id, install_static::GetBaseAppId. This varies for different Chrome channels (e.g., Canary vs. Stable) and different Chromium-based browsers (e.g., Chrome vs. Chromium).
The AUMI for a browser app has the format: <BaseAppId>.<app_name>[.<profile_name>]. profile_name is only appended when it's not the default profile.
The AUMI for a Chrome browser window has the format: <BaseAppId>[browser_suffix][.profile_name]. profile_name is only appended when it's not the default profile. browser_suffix is only appended to the BaseAppId if the installer has set the kRegisterChromeBrowserSuffix command line switch, e.g., on user-level installs.
Since AUMI‘s for browser and app windows include the profile_name, each profile’s windows will be grouped together on the taskbar.
shell_integration_win.cc has a function GetExpectedAppId to determine what the AUMI for a shortcut should be. It also has a function MigrateTaskbarPins to migrate pinned taskbar icons if the AUMI's need to change.
When the user has more than one profile, the shortcuts are renamed to include the profile name, e.g., Chrome.lnk becomes <profile name> - Chrome. The shortcut icons, both desktop and taskbar, are badged with their profile icon. This badged icon is also used in the tab preview for a Chrome window.
To dump a taskbar icon's properties, run this command:
vpython3 \src\chromium\src\chrome\installer\tools\shortcut_properties.py \
--dump-all \
"%APPDATA%\Microsoft\Internet Explorer\Quick Launch\User Pinned\TaskBar"
This shows you the properties of all the taskbar pinned icons. If the taskbar icon is in a subdirectory of ImplicitApps, pass that directory to shortcut_properties.py.