There are 5 parts to this study for which the primary objectives are to evaluate safety, tolerability, and pharmacokinetics (PK) of navitoclax when administered alone (Part 1) or when administered in combination with ruxolitinib (Part 2). In Part 2, participants must have been receiving a stable dose of ruxolitinib therapy for at least 12 weeks prior to study enrollment. In Part 3, all eligible participants will receive navitoclax, with the primary objective being to evaluate potential navitoclax effect on QTc prolongation. In Part 4, effect of navitoclax is evaluated on the PK, safety, and tolerability of a single dose of celecoxib. In Part 5, all eligible participants will receive ruxolitinib twice daily and navitoclax once daily for drug-drug interaction (DDI) assessment, followed by continued administration of navitoclax in combination with ruxolitinib.