added loading screen

src/main.cpp

@@ -30,13 +30,22 @@ int main(int argc, char** argv) {
   // declining must cost no buffer and no socket, and nothing below has to cope with "no source".
   // `--open` is the other answer to that question, and the chooser knows it: its Waveforms page
   // fills `open_waveform` in instead of the connection fields.
-  if (!cfg.server_specified && cfg.open_waveform.empty() && !udpscope::choose_source(cfg)) return 0;
+  const bool asked = !cfg.server_specified && cfg.open_waveform.empty();
+  if (asked && !udpscope::choose_source(cfg)) return 0;
   // A source named on the command line skips the chooser, but its *stored* configuration still
   // applies: the buffer size, the clock and the capture ring are decided here, before the model --
   // which is the only moment they can be -- and a saved host whose buffer was ignored because the
   // address came from the command line is an edit that looks like it never took effect.
   if (cfg.server_specified) udpscope::apply_stored_settings(cfg);
 
+  // Everything from here to `gtk_window_present()` inside `run_ui()` runs with no main loop: the
+  // buffer is allocated, a recording is read, then the whole UI is built. The chooser's window is
+  // already gone, so without this the screen is empty for all of it -- "the dialog disappeared and
+  // nothing happened", which at a few gigabytes of buffer is ten seconds long. Only on the path
+  // that *asked*: a source named with `--server` had no window to lose, and the status line after
+  // the present is the right place to say what such a run is waiting for.
+  if (asked) udpscope::loading_show(cfg);
+
   udpscope::ScopeModel model(cfg);
   udpscope::RowSet rows;
   rows.set_row_count(cfg.plots);