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 MF (PPV-MF) or post-essential thrombocytopenia MF (PET-MF) as
             defined by the 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 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, and are intolerant,
                  resistant, refractory or lost response to the JAKi.

          -  Segment B:

               -  Currently receiving ruxolitinib AND

               -  Willingness to reduce ruxolitinib 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 therapy;

                    2. < 24 weeks duration of current ruxolitinib course with documented
                       resistance, refractories, or loss of response, as defined by any of the
                       following:

                         -  Appearance of new splenomegaly that is palpable to at least 5 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 the 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 (BCL-2)
                  and/or BCL- XL inhibitor, including navitoclax.

          -  Segment D:

               -  Prior exposure to JAKi and/or any BET inhibitor.