Safety and Tolerability Study of Mivebresib Tablet Alone or in Combination With Ruxolitinib Tablet or Navitoclax Tablet in Adult Participants With Myelofibrosis

Inclusion Criteria: * Laboratory values indicative of adequate bone marrow, renal, and hepatic function meeting protocol criteria * Completion of the Myelofibrosis System Assessment Form (MFSAF) on at least 4 out of the 7 days prior to Day 1 with at least 2 symptoms with a score \>=3 or a total score of \>=10. * Documented diagnosis of intermediate or high-risk primary myelofibrosis (PMF), post-polycythemia vera myelofibrosis (PPV-MF) or post-essential thrombocytopenia myelofibrosis (PET-MF) as defined by World Health Organization (WHO). * Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1. * Intermediate - 2, or High-Risk disease as defined by the Dynamic International Prognostic Scoring System (For Segment A only, Intermediate - 1 with palpable splenomegaly \>=5 centimeters \[cm\] below costal margin are also eligible). * Splenomegaly defined as spleen palpation measurement \>= 5 centimeters (cm) below costal margin or spleen volume \>= 450 cubic cms as assessed by Magnetic Resonance Imaging (MRI) or Computed Tomography (CT) scan (for Segments A and c, baseline spleen assessment must be obtained \> 7 days after discontinuation of most recent Myelofibrosis (MF) therapy. If possible, this assessment should occur within 10 days of Cycle 1 Day 1). Segment-Specific Prior Therapy Criteria: * Segment A: --Prior exposure to one or more Janus Kinase Inhibitors (JAKi), the most recent of which was discontinued \> 28 days prior to Cycle 1 Day 1. * Segment B: * Currently receiving ruxolitinib; AND * Willingness to reduce dose (if on a higher dose); and on a stable dose for 14 days or longer prior to Cycle 1 Day 1; AND * At least one of the following criteria (a, b, or c): 1. \>= 24 weeks duration of current ruxolitinib course, with evidence of disease that is resistant, refractory, or has lost response to ruxolitinib monotherapy; 2. \< 24 weeks duration of current ruxolitinib course with documented disease progression as defined by any of the following: * Appearance of new splenomegaly that is palpable to at least 5 centimeters (cm) below the left costal margin (LCM), in participants with no evidence of splenomegaly prior to the initiation of ruxolitinib. * 100% increase in the palpable distance below the LCM, in participants with measurable spleen distance 5 - 10 cm prior to the initiation of ruxolitinib. * 50% increase in the palpable distance below the LCM, in participants with measurable spleen \> 10 cm prior to the initiation of ruxolitinib. * A spleen volume increase \>= 25% (as assessed by MRI or CT) in participants with a spleen volume assessment available prior to the initiation of ruxolitinib. 3. Prior treatment with ruxolitinib for \>= 28 days complicated by any of the following: * Development of red blood cell transfusion requirement (at least 2 units/month for 2 months). * Grade \>= 3 adverse events of neutropenia and/or anemia while on ruxolitinib treatment, with improvement or resolution upon dose reduction. * Segment C: * Prior exposure to one or more JAKi (the most recent of which was discontinued \> 28 days prior to Cycle 1 Day 1), and are intolerant, resistant, refractory or lost response to teh JAKi. Exclusion Criteria: Segment-Specific Prior Therapy Criteria: * Segment A: --Prior exposure to one or more Bromodomain and Extra Terminal (BET) inhibitors. * Segment B: --Prior exposure to one or more BET inhibitors. * Segment C: --Prior exposure to one or more BET inhibitors and/or any B-Cell Lymphoma 2 (BCL2) and/or B-Cell Lymphoma XL (BCLXL) inhibitor, including navitoclax. * Segment D: * Prior exposure to JAKi and/or any BET inhibitor.