feat: a reserved hole is shaded, so a sub-pixel gap is still a report

src/ui.cpp

@@ -6292,6 +6292,9 @@ gboolean tick(gpointer user) {
                                                 : model.display_epoch();
     }
     if (model.display_epoch() != want_epoch) model.set_display_epoch(want_epoch);
+    // The band last, after the epoch it is expressed in: it reads `model.holes()`, which the
+    // acquisition thread mutates, so it belongs under this lock and not after it.
+    app->rows->set_invalid_spans(model, display);
   }
 
   // The gesture probe's verdicts, one frame after the ask and after this frame's clamp: a