Bound live timeline rendering and simplify the Waterfall key

Height-constrained Gantts render and animate all100 lanes because windowing previously began only above100. Window every constrained timeline while preserving visible motion, full scroll height, selection and keyboard navigation. On the same Mac Retina native-wheel workload, the candidate measured59.59–59.85fps across eight theme/size/recording states, versus the retained53.96–57.51fps baseline; candidate windows had zero frames over50ms and zero long tasks.

Run ID: run_77a0e50b2c0abde48adf048324f3c2e0