layout editor: in-between split buttons sit above the divider handle; doc nits

src/ui.cpp

@@ -666,7 +666,10 @@ void sync_layout_editor(App& app) {
       gtk_widget_set_margin_start(
           b, static_cast<int>(std::clamp(cx, 0.0,
                                          std::max(0.0, static_cast<double>(gtk_widget_get_width(app.area_widget)) - kLayoutButtonPx)) + 0.5));
-      gtk_widget_set_margin_top(b, static_cast<int>(std::clamp(row_cy - half, 0.0, height - kLayoutButtonPx) + 0.5));
+      // An in-between `+` sits above the divider's resize handle (centred on the same point), or the
+      // handle, added later, would cover it and "split here" there could not be clicked.
+      const double top = pos > 0 && pos < n ? row_cy - half - kLayoutButtonPx - 2.0 : row_cy - half;
+      gtk_widget_set_margin_top(b, static_cast<int>(std::clamp(top, 0.0, height - kLayoutButtonPx) + 0.5));
     } else if (resize) {
       // Row resize handle: placed on the boundary between figure rows.
       const int idx = row_key(b);