docs: the readouts strip hides when empty

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ui_app.hpp

@@ -611,7 +611,8 @@ struct App {
   /// One value label per measurement, in `measures` order (the dock's copy of `measure_labels`).
   std::vector<GtkWidget*> dock_measure_labels;
   /// The readouts strip under the plot (`main-window.ui` `readouts_strip`, `src/ui_dock.cpp`): shown
-  /// whenever the dock is not beside the plot (`!show-sidebar || collapsed`). Its value labels mirror the dock's one for one -- `strip_cursor_deltas` the
+  /// whenever the dock is not beside the plot (`!show-sidebar || collapsed`) *and* there is something to
+  /// read (the cursors on, or a measurement; `sync_readouts_strip()`). Its value labels mirror the dock's one for one -- `strip_cursor_deltas` the
   /// `d` cell of each `dock_cursor_lines` entry, `strip_measure_labels` each of `dock_measure_labels` --
   /// and `strip_cursor_who` the cursor lines' `who` cells, all written from the dock setters' arguments.
   GtkWidget* readouts_strip = nullptr;
@@ -1801,7 +1802,8 @@ void dock_measure_value(App& app, std::size_t i, const std::string& text);
 /// The readouts strip under the plot (`src/ui_dock.cpp`): the third reader of the same strings. It is
 /// filled by the `dock_*` rebuilds and written by the `dock_*` setters above (so nothing here formats),
 /// and shown whenever the dock is not beside the plot (`!show-sidebar || collapsed`, a collapsed dock being
-/// an overlay on it): `sync_readouts_strip()` is called by the tick after the show-sidebar reconciliation
+/// an overlay on it) and there is something to read (cursors on or a measurement; each group follows its own
+/// feature, and with neither the strip is hidden): `sync_readouts_strip()` is called by the tick after the show-sidebar reconciliation
 /// and before `set_collapsed()`, so it reads the previous frame's `collapsed`. `dock_set_cursors_sensitive()` is the one writer of the cursor readouts'
 /// sensitivity, for the dock's grid and the strip's chips together.
 void sync_readouts_strip(App& app);