[client] Ship a legacy GTK3 UI package for distros without WebKitGTK 6.0

client/ui/xembed_host_gtk3_linux.go

@@ -0,0 +1,40 @@
+//go:build linux && gtk3 && !(linux && 386)
+
+package main
+
+import (
+	"errors"
+
+	"github.com/godbus/dbus/v5"
+)
+
+// The legacy GTK3 / WebKit2GTK 4.1 build (-tags gtk3) drops the in-process
+// XEmbed StatusNotifierWatcher entirely. The real implementation
+// (xembed_host_linux.go + xembed_tray_linux.c) links GTK4 and uses GTK4-only
+// popup-menu APIs that have no drop-in GTK3 equivalent, so rather than port the
+// C layer we stub the host out on gtk3 builds. The tray still works on every
+// desktop that ships its own StatusNotifierWatcher (KDE, GNOME+AppIndicator,
+// Cinnamon/xapp, XFCE, …); only the minimal-WM fallback (Fluxbox/OpenBox/i3/
+// dwm/vanilla GNOME) is unavailable on gtk3 packages. See LINUX-TRAY.md.
+
+// xembedHost is a placeholder so the package compiles on gtk3 builds; the real
+// type (with X11/GTK4 state) lives in xembed_host_linux.go. It is never
+// instantiated here because xembedTrayAvailable always reports false.
+type xembedHost struct{}
+
+// run satisfies the call in tray_watcher_linux.go; unreachable on gtk3 because
+// newXembedHost never returns a non-nil host.
+func (*xembedHost) run() {}
+
+// xembedTrayAvailable always reports false on gtk3 builds, so the watcher probe
+// loop in startStatusNotifierWatcher exits immediately and newXembedHost is
+// never reached. recenter_linux.go's predicate becomes a harmless no-op too.
+func xembedTrayAvailable() bool {
+	return false
+}
+
+// newXembedHost exists only to satisfy the reference in tray_watcher_linux.go;
+// it is unreachable because xembedTrayAvailable returns false on gtk3.
+func newXembedHost(conn *dbus.Conn, busName string, objPath dbus.ObjectPath) (*xembedHost, error) {
+	return nil, errors.New("xembed host unsupported on gtk3 build")
+}

client/ui/xembed_host_linux.go

@@ -1,4 +1,4 @@
-//go:build linux && !(linux && 386)
+//go:build linux && !gtk3 && !(linux && 386)
 
 package main
 

client/ui/xembed_tray_linux.c

@@ -1,3 +1,5 @@
+//go:build linux && !gtk3 && !(linux && 386)
+
 #include "xembed_tray_linux.h"
 
 #include <X11/Xatom.h>