added loading screen

src/ui.cpp

@@ -33,6 +33,7 @@
 #include "model.hpp"
 #include "plot_host.hpp"
 #include "search.hpp"
+#include "source_dialog.hpp"
 
 #include <adwaita.h>
 #include <gtk/gtk.h>
@@ -4838,6 +4839,12 @@ void on_activate(GtkApplication* application, gpointer user) {
 
   gtk_window_present(GTK_WINDOW(app->window));
 
+  // The loading window covered the gap from the chooser to here -- the buffer, the file read and
+  // the whole of the construction above, none of which returns to a main loop. It goes down only
+  // now, *after* the present: destroying it before would leave the screen empty again for the
+  // frame or two the real window takes to map. A no-op when there was no chooser.
+  loading_hide();
+
   // What the window is waiting for, said *before* the first tick. The stream's first CONFIG plans
   // the buffer on the producer thread, which holds the model's mutex for the whole of it -- at a few
   // gigabytes that is around a second per gigabyte, and the frame tick cannot read the model while