Bug 1929474 - [2/2] Document (some) other uses of Win10 tablet mode r=win-reviewers,firefox-desktop-core-reviewers ,handyman,mossop

Document remaining uses of Win10 tablet-mode detection which do not fall
under other bugs.

In particular, we leave alone:
  * `IsTabletDevice` in WinUtils.cpp. This falls under bug 1735765.
  * Uses in WinIMEHandler.{h,cpp}. These fall under bug 1929630.

No functional changes.

Differential Revision: https://phabricator.services.mozilla.com/D228206

browser/components/BrowserContentHandler.sys.mjs

@@ -1585,7 +1585,9 @@ nsDefaultCommandLineHandler.prototype = {
         cmdLine.state != Ci.nsICommandLine.STATE_INITIAL_LAUNCH &&
         lazy.WindowsUIUtils.inWin10TabletMode
       ) {
-        // In windows 10 tablet mode, do not create a new window, but reuse the existing one.
+        // In Win10's tablet mode, do not create a new window, but reuse the
+        // existing one. (Win11's tablet mode is still windowed and has no need
+        // for this workaround.)
         let win = lazy.BrowserWindowTracker.getTopWindow();
         if (win) {
           win.focus();

xpfe/appshell/AppWindow.cpp

@@ -1821,6 +1821,8 @@ nsresult AppWindow::MaybeSaveEarlyWindowPersistentValues(
     nsCOMPtr<nsIWindowsUIUtils> uiUtils(
         do_GetService("@mozilla.org/windows-ui-utils;1"));
     if (!NS_WARN_IF(!uiUtils)) {
+      // We switch to the touch-optimized layout in both Win10 and Win11 tablet-
+      // modes, since only the input mechanism is relevant. (See bug 1819421.)
       if (IsWin11OrLater()) {
         uiUtils->GetInWin11TabletMode(&isInTabletMode);
       } else {