WindowServer+LibGUI: Allow changing whether windows have alpha channels.

Use this in Terminal to tell the window server to not bother with the alpha
channel in the backing store if we're running without transparency.
Semi-transparent terminals look neat but they slow everything down, so this
keeps things fast while making it easy to switch to the flashy mode. :^)

Applications/Terminal/Terminal.cpp

@@ -891,6 +891,7 @@ void Terminal::set_opacity(float opacity)
 {
     if (m_opacity == opacity)
         return;
+    window()->set_has_alpha_channel(opacity < 1);
     m_opacity = opacity;
     force_repaint();
 }

Applications/Terminal/main.cpp

@@ -116,7 +116,7 @@ int main(int argc, char** argv)
     };
 
     slider->set_range(0, 100);
-    slider->set_value(80);
+    slider->set_value(100);
 
     auto menubar = make<GMenuBar>();
 

LibGUI/GEventLoop.cpp

@@ -376,6 +376,9 @@ bool GEventLoop::wait_for_specific_event(WSAPI_ServerMessage::Type type, WSAPI_S
         FD_ZERO(&rfds);
         FD_SET(s_event_fd, &rfds);
         int rc = select(s_event_fd + 1, &rfds, nullptr, nullptr, nullptr);
+        if (rc < 0) {
+            perror("select");
+        }
         ASSERT(rc > 0);
         ASSERT(FD_ISSET(s_event_fd, &rfds));
         bool success = drain_messages_from_server();

LibGUI/GWindow.cpp

@@ -386,8 +386,23 @@ void GWindow::set_automatic_cursor_tracking_widget(GWidget* widget)
 
 void GWindow::set_has_alpha_channel(bool value)
 {
-    ASSERT(!m_window_id);
+    if (m_has_alpha_channel == value)
+        return;
     m_has_alpha_channel = value;
+    if (!m_window_id)
+        return;
+
+    m_pending_paint_event_rects.clear();
+    m_back_bitmap = nullptr;
+    m_front_bitmap = nullptr;
+
+    WSAPI_ClientMessage message;
+    message.type = WSAPI_ClientMessage::Type::SetWindowHasAlphaChannel;
+    message.window_id = m_window_id;
+    message.value = value;
+    GEventLoop::current().sync_request(message, WSAPI_ServerMessage::DidSetWindowHasAlphaChannel);
+
+    update();
 }
 
 void GWindow::set_double_buffering_enabled(bool value)

Servers/WindowServer/WSAPITypes.h

@@ -103,6 +103,7 @@ struct WSAPI_ServerMessage {
         DidSetWindowBackingStore,
         DidSetWallpaper,
         DidGetWallpaper,
+        DidSetWindowHasAlphaChannel,
         ScreenRectChanged,
         WM_WindowRemoved,
         WM_WindowStateChanged,
@@ -218,6 +219,7 @@ struct WSAPI_ClientMessage {
         PopupMenu,
         DismissMenu,
         SetWindowIcon,
+        SetWindowHasAlphaChannel,
     };
     Type type { Invalid };
     int window_id { -1 };

Servers/WindowServer/WSClientConnection.cpp

@@ -629,6 +629,24 @@ void WSClientConnection::handle_request(const WSAPISetWindowOverrideCursorReques
     window.set_override_cursor(WSCursor::create(request.cursor()));
 }
 
+void WSClientConnection::handle_request(const WSAPISetWindowHasAlphaChannelRequest& request)
+{
+    int window_id = request.window_id();
+    auto it = m_windows.find(window_id);
+    if (it == m_windows.end()) {
+        post_error("WSAPISetWindowHasAlphaChannelRequest: Bad window ID");
+        return;
+    }
+    auto& window = *(*it).value;
+    window.set_has_alpha_channel(request.value());
+
+    WSAPI_ServerMessage response;
+    response.type = WSAPI_ServerMessage::Type::DidSetWindowHasAlphaChannel;
+    response.window_id = window_id;
+    response.value = request.value();
+    post_message(response);
+}
+
 void WSClientConnection::handle_request(const WSWMAPISetActiveWindowRequest& request)
 {
     auto* client = WSClientConnection::from_client_id(request.target_client_id());
@@ -747,6 +765,8 @@ void WSClientConnection::on_request(const WSAPIClientRequest& request)
         return handle_request(static_cast<const WSAPIPopupMenuRequest&>(request));
     case WSEvent::APIDismissMenuRequest:
         return handle_request(static_cast<const WSAPIDismissMenuRequest&>(request));
+    case WSEvent::APISetWindowHasAlphaChannelRequest:
+        return handle_request(static_cast<const WSAPISetWindowHasAlphaChannelRequest&>(request));
     default:
         break;
     }

Servers/WindowServer/WSClientConnection.h

@@ -77,6 +77,7 @@ private:
     void handle_request(const WSWMAPIStartWindowResizeRequest&);
     void handle_request(const WSAPIPopupMenuRequest&);
     void handle_request(const WSAPIDismissMenuRequest&);
+    void handle_request(const WSAPISetWindowHasAlphaChannelRequest&);
 
     void post_error(const String&);
 

Servers/WindowServer/WSEvent.h

@@ -60,6 +60,7 @@ public:
         APISetWallpaperRequest,
         APIGetWallpaperRequest,
         APISetWindowOverrideCursorRequest,
+        APISetWindowHasAlphaChannelRequest,
         WMAPISetActiveWindowRequest,
         WMAPISetWindowMinimizedRequest,
         WMAPIStartWindowResizeRequest,
@@ -387,6 +388,23 @@ private:
     WSStandardCursor m_cursor { WSStandardCursor::None };
 };
 
+class WSAPISetWindowHasAlphaChannelRequest final : public WSAPIClientRequest {
+public:
+    explicit WSAPISetWindowHasAlphaChannelRequest(int client_id, int window_id, bool value)
+        : WSAPIClientRequest(WSEvent::APISetWindowHasAlphaChannelRequest, client_id)
+        , m_window_id(window_id)
+        , m_value(value)
+    {
+    }
+
+    int window_id() const { return m_window_id; }
+    bool value() const { return m_value; }
+
+private:
+    int m_window_id { 0 };
+    bool m_value { 0 };
+};
+
 class WSAPISetWallpaperRequest final : public WSAPIClientRequest {
 public:
     explicit WSAPISetWallpaperRequest(int client_id, String&& wallpaper)

Servers/WindowServer/WSEventLoop.cpp

@@ -244,6 +244,9 @@ bool WSEventLoop::on_receive_from_client(int client_id, const WSAPI_ClientMessag
     case WSAPI_ClientMessage::Type::SetWindowOverrideCursor:
         post_event(client, make<WSAPISetWindowOverrideCursorRequest>(client_id, message.window_id, (WSStandardCursor)message.cursor.cursor));
         break;
+    case WSAPI_ClientMessage::SetWindowHasAlphaChannel:
+        post_event(client, make<WSAPISetWindowHasAlphaChannelRequest>(client_id, message.window_id, message.value));
+        break;
     case WSAPI_ClientMessage::Type::WM_SetActiveWindow:
         post_event(client, make<WSWMAPISetActiveWindowRequest>(client_id, message.wm.client_id, message.wm.window_id));
         break;