diff --git a/sommelier-x11event-test.cc b/sommelier-x11event-test.cc
index f03ef5c..3e5a528 100644
--- a/sommelier-x11event-test.cc
+++ b/sommelier-x11event-test.cc
@@ -149,5 +149,87 @@ TEST_F(X11EventTest, PropertyNotifyStoresSteamId) {
   EXPECT_EQ(window->steam_game_id, steam_game_id);
 }
 
+TEST_F(X11EventTest, MapRequestParsesSteamIdFromClass) {
+  xcb.DelegateToFake();
+  sl_window* window = CreateWindowWithoutRole();
+  xcb.create_window(nullptr, 32, window->id, XCB_WINDOW_NONE, 0, 0, 800, 600, 0,
+                    XCB_WINDOW_CLASS_INPUT_OUTPUT, XCB_COPY_FROM_PARENT, 0,
+                    nullptr);
+  const char clazz[] = "steam_app_7890\0steam_app_7890";
+  xcb.change_property(nullptr, XCB_PROP_MODE_REPLACE, window->id,
+                      XCB_ATOM_WM_CLASS, XCB_ATOM_STRING, 8, sizeof(clazz),
+                      clazz);
+
+  xcb_map_request_event_t event;
+  event.response_type = XCB_MAP_REQUEST;
+  event.window = window->id;
+  sl_handle_map_request(&ctx, &event);
+
+  EXPECT_EQ(window->steam_game_id, 7890);
+}
+
+TEST_F(X11EventTest, MapRequestPrefersSteamGameIdOverClass) {
+  uint32_t steam_game_id = 123456;
+  xcb.DelegateToFake();
+  sl_window* window = CreateWindowWithoutRole();
+  xcb.create_window(nullptr, 32, window->id, XCB_WINDOW_NONE, 0, 0, 800, 600, 0,
+                    XCB_WINDOW_CLASS_INPUT_OUTPUT, XCB_COPY_FROM_PARENT, 0,
+                    nullptr);
+  const char clazz[] = "steam_app_7890\0steam_app_7890";
+  xcb.change_property(nullptr, XCB_PROP_MODE_REPLACE, window->id,
+                      XCB_ATOM_WM_CLASS, XCB_ATOM_STRING, 8, sizeof(clazz),
+                      clazz);
+  xcb.change_property(nullptr, XCB_PROP_MODE_REPLACE, window->id,
+                      ctx.atoms[ATOM_STEAM_GAME].value, XCB_ATOM_CARDINAL, 32,
+                      1, &steam_game_id);
+
+  // Act: map the window
+  xcb_map_request_event_t event;
+  event.response_type = XCB_MAP_REQUEST;
+  event.window = window->id;
+  sl_handle_map_request(&ctx, &event);
+
+  // Assert: Uses the ID from the STEAM_GAME property, not from WM_CLASS
+  EXPECT_EQ(window->steam_game_id, steam_game_id);
+}
+
+TEST_F(X11EventTest, PropertyNotifyParsesSteamIdFromClassAsFallback) {
+  uint32_t steam_game_id = 123456;
+  xcb.DelegateToFake();
+  sl_window* window = CreateWindowWithoutRole();
+  xcb.create_window(nullptr, 32, window->id, XCB_WINDOW_NONE, 0, 0, 800, 600, 0,
+                    XCB_WINDOW_CLASS_INPUT_OUTPUT, XCB_COPY_FROM_PARENT, 0,
+                    nullptr);
+
+  // Act: set STEAM_GAME to one ID, and WM_CLASS to specify another
+  xcb.change_property(nullptr, XCB_PROP_MODE_REPLACE, window->id,
+                      ctx.atoms[ATOM_STEAM_GAME].value, XCB_ATOM_CARDINAL, 32,
+                      1, &steam_game_id);
+  {
+    xcb_property_notify_event_t event;
+    event.response_type = XCB_PROPERTY_NOTIFY;
+    event.window = window->id;
+    event.atom = ctx.atoms[ATOM_STEAM_GAME].value;
+    event.state = XCB_PROPERTY_NEW_VALUE;
+    sl_handle_property_notify(&ctx, &event);
+  }
+
+  const char clazz[] = "steam_app_7890\0steam_app_7890";
+  xcb.change_property(nullptr, XCB_PROP_MODE_REPLACE, window->id,
+                      XCB_ATOM_WM_CLASS, XCB_ATOM_STRING, 8, sizeof(clazz),
+                      clazz);
+  {
+    xcb_property_notify_event_t event;
+    event.response_type = XCB_PROPERTY_NOTIFY;
+    event.window = window->id;
+    event.atom = XCB_ATOM_WM_CLASS;
+    event.state = XCB_PROPERTY_NEW_VALUE;
+    sl_handle_property_notify(&ctx, &event);
+  }
+
+  // Assert: STEAM_GAME is preferred, even though WM_CLASS was changed last.
+  EXPECT_EQ(window->steam_game_id, steam_game_id);
+}
+
 }  // namespace sommelier
 }  // namespace vm_tools
diff --git a/sommelier.cc b/sommelier.cc
index 46296b5..ebe31fe 100644
--- a/sommelier.cc
+++ b/sommelier.cc
@@ -12,6 +12,7 @@
 #include "xcb/xcb-shim.h"
 
 #include <assert.h>
+#include <cstdlib>
 #include <errno.h>
 #include <fcntl.h>
 #include <gbm.h>
@@ -97,6 +98,8 @@ struct sl_data_source {
 #define MIN_AURA_SHELL_VERSION 6
 #define MAX_AURA_SHELL_VERSION 38
 
+static const char STEAM_APP_CLASS_PREFIX[] = "steam_app_";
+
 int sl_open_wayland_socket(const char* socket_name,
                            struct sockaddr_un* addr,
                            int* lock_fd,
@@ -1125,12 +1128,24 @@ static const char* sl_decode_wm_class(struct sl_window* window,
   // WM_CLASS property contains two consecutive null-terminated strings.
   // These specify the Instance and Class names. If a global app ID is
   // not set then use Class name for app ID.
-  const char* value = static_cast<char*>(xcb_get_property_value(reply));
-  int value_length = xcb_get_property_value_length(reply);
+  const char* value = static_cast<char*>(xcb()->get_property_value(reply));
+  int value_length = xcb()->get_property_value_length(reply);
   int instance_length = strnlen(value, value_length);
   if (value_length > instance_length) {
     window->clazz = strndup(value + instance_length + 1,
                             value_length - instance_length - 1);
+
+    if (!window->steam_game_id) {
+      // If there's no known Steam Game ID for this window,
+      // attempt to parse one from the class name.
+      if (strncmp(window->clazz, STEAM_APP_CLASS_PREFIX,
+                  strlen(STEAM_APP_CLASS_PREFIX)) == 0) {
+        // atoi() returns 0 on error, in which case steam_game_id
+        // simply remains effectively unset.
+        window->steam_game_id =
+            atoi(window->clazz + strlen(STEAM_APP_CLASS_PREFIX));
+      }
+    }
     return window->clazz;
   }
   return nullptr;