The position is (slightly) broken on wayland/2K but the sizing is OK. So it looks like the positioning is wayland-related and the size is hidpi-related.

A workaround for the wayland positioning errors is to use GDK_BACKEND=x11 - see https://trac.wxwidgets.org/ticket/17702

cah@client1455:~/src/wxWidgets$ git diff
diff --git a/include/wx/unix/glegl.h b/include/wx/unix/glegl.h
index ccdd302e36..cbe3ffaf9d 100644
--- a/include/wx/unix/glegl.h
+++ b/include/wx/unix/glegl.h
@@ -122,6 +122,7 @@ public:
     wl_subcompositor *m_wlSubcompositor;
     wl_callback *m_wlFrameCallbackHandler;
     wl_egl_window *m_wlEGLWindow;
+    wl_subsurface *m_wlSubsurface;

 private:

@@ -132,7 +133,6 @@ private:
     unsigned long m_xwindow;
     wl_surface *m_wlSurface;
     wl_region *m_wlRegion;
-    wl_subsurface *m_wlSubsurface;

     // the global/default versions of the above
     static EGLConfig *ms_glEGLConfig;
diff --git a/src/unix/glegl.cpp b/src/unix/glegl.cpp
index fa4ad29b51..4930ee2960 100644
--- a/src/unix/glegl.cpp
+++ b/src/unix/glegl.cpp
@@ -444,6 +444,9 @@ static void gtk_glcanvas_size_callback(GtkWidget *widget,
     int scale = gtk_widget_get_scale_factor(widget);
     wl_egl_window_resize(win->m_wlEGLWindow, win->m_width * scale,
                          win->m_height * scale, 0, 0);
+    int x, y;
+    gdk_window_get_origin(win->GTKGetDrawingWindow(), &x, &y);
+    wl_subsurface_set_position(win->m_wlSubsurface, x, y);
 }

 } // extern "C"

Seems to help - bit of a hack though, especially setting up the position in response to a size callback. I tried to listen to configure-event but it never seemed to show up.

Added that patch to b73b43da73aeddf2ca0875a8a55022cf2f698622 in dcpomatic-infrastructure

ubuntu-16.04-64
centos-7-64
ubuntu-21.04-64
debian-10-64
mageia-8-64
fedora-33-64
fedora-34-64
debian-unstable-64
ubuntu-18.04-64/
ubuntu-21.10-64/
debian-9-64/
debian-11-64/
/ubuntu-20.04-64/
ubuntu-16.04-32/
centos-8-64