feat: Can a window always on top but behind the taskbar on Win32 (#18982)

For now it only adds the ability to place the window below
the task bar while still being always on top.
Previous behaviour was always showing the window above the task
bar when top is true. We keep this default behaviour, i.e. when
the 'level' parameter is omitted.

https://github.com/electron/electron/issues/18933

Notes: Can set a window always on top but behind the taskbar on Windows

docs/api/browser-window.md

@@ -1100,10 +1100,15 @@ On Linux always returns `true`.
 #### `win.setAlwaysOnTop(flag[, level][, relativeLevel])`
 
 * `flag` Boolean
-* `level` String (optional) _macOS_ - Values include `normal`, `floating`,
-  `torn-off-menu`, `modal-panel`, `main-menu`, `status`, `pop-up-menu`,
-  `screen-saver`, and ~~`dock`~~ (Deprecated). The default is `floating`. See the
-  [macOS docs][window-levels] for more details.
+* `level` String (optional) _macOS_ _Windows_ - Values include `normal`,
+  `floating`, `torn-off-menu`, `modal-panel`, `main-menu`, `status`,
+  `pop-up-menu`, `screen-saver`, and ~~`dock`~~ (Deprecated). The default is
+  `floating` when `flag` is true. The `level` is reset to `normal` when the
+  flag is false. Note that from `floating` to `status` included, the window is
+  placed below the Dock on macOS and below the taskbar on Windows. From
+  `pop-up-menu` to a higher it is shown above the Dock on macOS and above the
+  taskbar on Windows. See the [macOS docs][window-levels] for more details.
+
 * `relativeLevel` Integer (optional) _macOS_ - The number of layers higher to set
   this window relative to the given `level`. The default is `0`. Note that Apple
   discourages setting levels higher than 1 above `screen-saver`.

package.json

@@ -125,4 +125,4 @@
       "git add filenames.auto.gni"
     ]
   }
-}
+}

shell/browser/native_window_views.cc

@@ -13,6 +13,7 @@
 #include <utility>
 #include <vector>
 
+#include "base/stl_util.h"
 #include "base/strings/utf_string_conversions.h"
 #include "content/public/browser/browser_thread.h"
 #include "native_mate/dictionary.h"
@@ -69,6 +70,8 @@ namespace electron {
 namespace {
 
 #if defined(OS_WIN)
+const LPCWSTR kUniqueTaskBarClassName = L"Shell_TrayWnd";
+
 void FlipWindowStyle(HWND handle, bool on, DWORD flag) {
   DWORD style = ::GetWindowLong(handle, GWL_STYLE);
   if (on)
@@ -757,6 +760,19 @@ void NativeWindowViews::SetAlwaysOnTop(bool top,
     NativeWindow::NotifyWindowAlwaysOnTopChanged();
 
   widget()->SetAlwaysOnTop(top);
+
+#if defined(OS_WIN)
+  // Reset the placement flag.
+  behind_task_bar_ = false;
+  if (top) {
+    // On macOS the window is placed behind the Dock for the following levels.
+    // Re-use the same names on Windows to make it easier for the user.
+    static const std::vector<std::string> levels = {
+        "floating", "torn-off-menu", "modal-panel", "main-menu", "status"};
+    behind_task_bar_ = base::Contains(levels, level);
+  }
+#endif
+  MoveBehindTaskBarIfNeeded();
 }
 
 bool NativeWindowViews::IsAlwaysOnTop() {
@@ -1184,10 +1200,12 @@ void NativeWindowViews::OnWidgetActivationChanged(views::Widget* changed_widget,
   if (changed_widget != widget())
     return;
 
-  if (active)
+  if (active) {
+    MoveBehindTaskBarIfNeeded();
     NativeWindow::NotifyWindowFocus();
-  else
+  } else {
     NativeWindow::NotifyWindowBlur();
+  }
 
   // Hide menu bar when window is blured.
   if (!active && IsMenuBarAutoHide() && IsMenuBarVisible())
@@ -1346,6 +1364,17 @@ ui::WindowShowState NativeWindowViews::GetRestoredState() {
   return ui::SHOW_STATE_NORMAL;
 }
 
+void NativeWindowViews::MoveBehindTaskBarIfNeeded() {
+#if defined(OS_WIN)
+  if (behind_task_bar_) {
+    const HWND task_bar_hwnd = ::FindWindow(kUniqueTaskBarClassName, nullptr);
+    ::SetWindowPos(GetAcceleratedWidget(), task_bar_hwnd, 0, 0, 0, 0,
+                   SWP_NOMOVE | SWP_NOSIZE | SWP_NOACTIVATE);
+  }
+#endif
+  // TODO(julien.isorce): Implement X11 case.
+}
+
 // static
 NativeWindow* NativeWindow::Create(const mate::Dictionary& options,
                                    NativeWindow* parent) {

shell/browser/native_window_views.h

@@ -214,6 +214,9 @@ class NativeWindowViews : public NativeWindow,
   // Returns the restore state for the window.
   ui::WindowShowState GetRestoredState();
 
+  // Maintain window placement.
+  void MoveBehindTaskBarIfNeeded();
+
   std::unique_ptr<RootView> root_view_;
 
   // The view should be focused by default.
@@ -280,6 +283,9 @@ class NativeWindowViews : public NativeWindow,
   bool forwarding_mouse_messages_ = false;
   HWND legacy_window_ = NULL;
   bool layered_ = false;
+
+  // Set to true if the window is always on top and behind the task bar.
+  bool behind_task_bar_ = false;
 #endif
 
   // Handles unhandled keyboard messages coming back from the renderer process.