[dagit] Prevent solid selection from overlapping RunRoot header
Long-standing issue. When collapsing the Gantt chart panel on a Run, the solid selection input overlaps the header. Do a little z-index skirmishing to repair.
Test Plan: View Run, collapse Gantt panel. Verify that the solid selection input is not visible.
Reviewers: bengotow, prha, sashank
Reviewed By: bengotow
Differential Revision: https://dagster.phacility.com/D7608