Add tooltip to timeline selector

crates/re_time_panel/src/time_control_ui.rs

@@ -41,7 +41,26 @@ impl TimeControlUi {
                             time_control.set_timeline(*timeline);
                         }
                     }
-                });
+                })
+                .response
+                .on_hover_ui(|ui| {
+                    re_ui::markdown_ui(
+                        ui,
+                        egui::Id::new("timeline_selector_tooltip"),
+                        r"
+Select timeline.
+
+Each piece of logged data is associated with one or more timelines.
+
+The logging SDK always creates two timelines for you:
+* `log_tick` - a sequence timeline with the sequence number of the log call
+* `log_time` - a temporal timeline with the time of the log call
+
+You can also define your own timelines, e.g. for sensor time or camera frame number.
+"
+                        .trim(),
+                    );
+                })
         });
     }
 

crates/re_ui/src/lib.rs

@@ -1207,6 +1207,8 @@ pub fn help_hover_button(ui: &mut egui::Ui) -> egui::Response {
 }
 
 /// Show some markdown
+///
+/// The `id` should be globally unique and persistent.
 pub fn markdown_ui(ui: &mut egui::Ui, id: egui::Id, markdown: &str) {
     use parking_lot::Mutex;
     use std::sync::Arc;