Show a Linux tray icon's title as its label, not in the tooltip

The StatusNotifierItem put SetTitle's text into the ToolTip heading, and into
its body when no tooltip was set, so hosts showed the title on hover. The
ToolTip now carries the tooltip alone; the title is the XAyatanaLabel (with
XAyatanaNewLabel), which GNOME's AppIndicator extension draws next to the
icon as macOS draws the title, and as libayatana's app_indicator_set_label
did; KDE and most bars ignore it. Title names the application, as the SNI
spec has it, which hosts list the item by and show for an empty tooltip.

Author: lijy91

src/platform/linux/tray_icon_linux.cpp

@@ -112,6 +112,8 @@ static const char kSniIntrospectionXml[] =
     "    <property name='ToolTip'            type='(sa(iiay)ss)' access='read'/>"
     "    <property name='ItemIsMenu'         type='b'        access='read'/>"
     "    <property name='Menu'               type='o'        access='read'/>"
+    "    <property name='XAyatanaLabel'      type='s'        access='read'/>"
+    "    <property name='XAyatanaLabelGuide' type='s'        access='read'/>"
     "    <method name='ContextMenu'>"
     "      <arg type='i' direction='in' name='x'/>"
     "      <arg type='i' direction='in' name='y'/>"
@@ -133,6 +135,10 @@ static const char kSniIntrospectionXml[] =
     "    <signal name='NewAttentionIcon'/>"
     "    <signal name='NewOverlayIcon'/>"
     "    <signal name='NewToolTip'/>"
+    "    <signal name='XAyatanaNewLabel'>"
+    "      <arg type='s' name='label'/>"
+    "      <arg type='s' name='guide'/>"
+    "    </signal>"
     "    <signal name='NewStatus'>"
     "      <arg type='s' name='status'/>"
     "    </signal>"
@@ -624,8 +630,19 @@ class TrayIcon::Impl {
     if (g_strcmp0(property_name, "Id") == 0)
       return g_variant_new_string(self->identifier_.c_str());
 
-    if (g_strcmp0(property_name, "Title") == 0)
+    // The SNI Title names the application; hosts list the item by it and fall
+    // back to it for an empty tooltip. SetTitle's text is the Ayatana label below.
+    if (g_strcmp0(property_name, "Title") == 0) {
+      const char* name = g_get_application_name();
+      return g_variant_new_string(name ? name : self->identifier_.c_str());
+    }
+
+    // Drawn next to the icon by hosts that know it (GNOME's AppIndicator
+    // extension), as macOS draws the title; KDE and most bars ignore it.
+    if (g_strcmp0(property_name, "XAyatanaLabel") == 0)
       return g_variant_new_string(self->title_.value_or("").c_str());
+    if (g_strcmp0(property_name, "XAyatanaLabelGuide") == 0)
+      return g_variant_new_string("");
 
     if (g_strcmp0(property_name, "Status") == 0)
       return g_variant_new_string(self->visible_ ? "Active" : "Passive");
@@ -651,10 +668,10 @@ class TrayIcon::Impl {
     if (g_strcmp0(property_name, "AttentionMovieName") == 0) return g_variant_new_string("");
 
     if (g_strcmp0(property_name, "ToolTip") == 0) {
-      // (sa(iiay)ss): iconName, iconPixmap[], title, description
-      const std::string& tip = self->tooltip_.value_or(self->title_.value_or(""));
+      // (sa(iiay)ss): iconName, iconPixmap[], title, description. The tooltip
+      // is the heading; hosts show an empty one as the item's Title.
       return g_variant_new("(s@a(iiay)ss)", "", PixbufToSniIconPixmaps(nullptr),
-                           self->title_.value_or("").c_str(), tip.c_str());
+                           self->tooltip_.value_or("").c_str(), "");
     }
 
     if (g_strcmp0(property_name, "ItemIsMenu") == 0)
@@ -1095,7 +1112,8 @@ std::shared_ptr<View> TrayIcon::GetContentView() const {
 
 void TrayIcon::SetTitle(std::optional<std::string> title) {
   pimpl_->title_ = title;
-  pimpl_->EmitSignal("NewTitle");
+  pimpl_->EmitSignal("XAyatanaNewLabel",
+                     g_variant_new("(ss)", pimpl_->title_.value_or("").c_str(), ""));
 }
 
 std::optional<std::string> TrayIcon::GetTitle() {