diff --git a/.gitignore b/.gitignore
index c74e7438a387..bb332f00e77b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -54,6 +54,8 @@ Thumbs.db
 ###################################
 lib/matplotlib/mpl-data/matplotlib.conf
 lib/matplotlib/mpl-data/matplotlibrc
+tutorials/intermediate/CL01.png
+tutorials/intermediate/CL02.png
 
 # Documentation generated files #
 #################################
diff --git a/INSTALL.rst b/INSTALL.rst
index 9e7528b68bd0..087ff0dd2e3a 100644
--- a/INSTALL.rst
+++ b/INSTALL.rst
@@ -112,13 +112,6 @@ file will be particularly useful to those packaging Matplotlib.
 
 .. _setup.cfg: https://raw.githubusercontent.com/matplotlib/matplotlib/master/setup.cfg.template
 
-If you have installed prerequisites to nonstandard places and need to
-inform Matplotlib where they are, edit ``setupext.py`` and add the base
-dirs to the ``basedir`` dictionary entry for your ``sys.platform``;
-e.g., if the header of some required library is in
-``/some/path/include/someheader.h``, put ``/some/path`` in the
-``basedir`` list for your platform.
-
 .. _install_requirements:
 
 Dependencies
diff --git a/doc/_static/mpl.css b/doc/_static/mpl.css
index ac4bde0486f9..8cdc2e5668d9 100644
--- a/doc/_static/mpl.css
+++ b/doc/_static/mpl.css
@@ -594,6 +594,10 @@ span.linkdescr {
     font-size: 90%;
 }
 
+.highlight span.c1 span.highlighted {
+    background-color: #fce5a6;
+}
+
 ul.search {
     margin: 10px 0 0 20px;
     padding: 0;
diff --git a/doc/api/ticker_api.rst b/doc/api/ticker_api.rst
index 8da9ac2988f0..652050dafedc 100644
--- a/doc/api/ticker_api.rst
+++ b/doc/api/ticker_api.rst
@@ -6,3 +6,7 @@
    :members:
    :undoc-members:
    :show-inheritance:
+
+
+.. inheritance-diagram:: matplotlib.ticker
+   :parts: 1
diff --git a/doc/devel/documenting_mpl.rst b/doc/devel/documenting_mpl.rst
index 26b019af7ea7..98d2fd23ad88 100644
--- a/doc/devel/documenting_mpl.rst
+++ b/doc/devel/documenting_mpl.rst
@@ -500,15 +500,8 @@ section of the docstring.
 rcParams
 ~~~~~~~~
 rcParams can be referenced with the custom ``:rc:`` role:
-:literal:`:rc:\`foo\`` yields ``rcParams["foo"]``. Use `= [default-val]`
-to indicate the default value of the parameter. The default value should be
-literal, i.e. enclosed in double backticks. For simplicity these may be
-omitted for string default values.
-
-.. code-block:: rst
-
-  If not provided, defaults to :rc:`figure.figsize` = ``[6.4, 4.8]``.
-  If not provided, defaults to :rc:`figure.facecolor` = 'w'.
+:literal:`:rc:\`foo\`` yields ``rcParams["foo"] = 'default'``, which is a link
+to the :file:`matplotlibrc` file description.
 
 Deprecated formatting conventions
 ---------------------------------
diff --git a/doc/index.rst b/doc/index.rst
index bfb42dc74cca..1abbd0092ebe 100644
--- a/doc/index.rst
+++ b/doc/index.rst
@@ -182,12 +182,25 @@ Open source
     style="float:right; margin-left:20px" />
    </a>
 
+
+Matplotlib is a Sponsored Project of NumFOCUS, a 501(c)(3) nonprofit
+charity in the United States. NumFOCUS provides Matplotlib with
+fiscal, legal, and administrative support to help ensure the health
+and sustainability of the project. Visit `numfocus.org <nf>`_ for more
+information.
+
+Donations to Matplotlib are managed by NumFOCUS. For donors in the
+United States, your gift is tax-deductible to the extent provided by
+law. As with any donation, you should consult with your tax adviser
+about your particular tax situation.
+
 Please consider `donating to the Matplotlib project <donating_>`_ through
 the Numfocus organization or to the `John Hunter Technology Fellowship
 <jdh-fellowship_>`_.
 
 .. _donating: https://numfocus.salsalabs.org/donate-to-matplotlib/index.html
 .. _jdh-fellowship: https://www.numfocus.org/programs/john-hunter-technology-fellowship/
+.. _nf: https://numfocus.org
 
 The :doc:`Matplotlib license <users/license>` is based on the `Python Software
 Foundation (PSF) license <psf-license_>`_.
diff --git a/doc/sphinxext/custom_roles.py b/doc/sphinxext/custom_roles.py
index 2232f1032424..88dddb6a2e4b 100644
--- a/doc/sphinxext/custom_roles.py
+++ b/doc/sphinxext/custom_roles.py
@@ -1,16 +1,19 @@
 from docutils import nodes
 from os.path import sep
+from matplotlib import rcParamsDefault
 
 
 def rcparam_role(name, rawtext, text, lineno, inliner, options={}, content=[]):
-    rendered = nodes.Text('rcParams["{}"]'.format(text))
+    param = rcParamsDefault.get(text)
+    rendered = nodes.Text(f'rcParams["{text}"] = {param!r}')
 
     source = inliner.document.attributes['source'].replace(sep, '/')
     rel_source = source.split('/doc/', 1)[1]
 
     levels = rel_source.count('/')
     refuri = ('../' * levels +
-              'tutorials/introductory/customizing.html#matplotlib-rcparams')
+              'tutorials/introductory/customizing.html' +
+              f"?highlight={text}#a-sample-matplotlibrc-file")
 
     ref = nodes.reference(rawtext, rendered, refuri=refuri)
     return [nodes.literal('', '', ref)], []
diff --git a/examples/lines_bars_and_markers/horizontal_barchart_distribution.py b/examples/lines_bars_and_markers/horizontal_barchart_distribution.py
index 73367e4b1d60..c851997ad29a 100644
--- a/examples/lines_bars_and_markers/horizontal_barchart_distribution.py
+++ b/examples/lines_bars_and_markers/horizontal_barchart_distribution.py
@@ -70,6 +70,7 @@ def survey(results, category_names):
 
 
 survey(results, category_names)
+plt.show()
 
 #############################################################################
 #
diff --git a/lib/matplotlib/_constrained_layout.py b/lib/matplotlib/_constrained_layout.py
index a2a70fdc0212..a79b61be6754 100644
--- a/lib/matplotlib/_constrained_layout.py
+++ b/lib/matplotlib/_constrained_layout.py
@@ -280,7 +280,11 @@ def _make_layout_margins(ax, renderer, h_pad, w_pad):
     invTransFig = fig.transFigure.inverted().transform_bbox
     pos = ax.get_position(original=True)
     tightbbox = ax.get_tightbbox(renderer=renderer)
-    bbox = invTransFig(tightbbox)
+    if tightbbox is None:
+        bbox = pos
+    else:
+        bbox = invTransFig(tightbbox)
+
     # this can go wrong:
     if not (np.isfinite(bbox.width) and np.isfinite(bbox.height)):
         # just abort, this is likely a bad set of co-ordinates that
diff --git a/lib/matplotlib/axes/_axes.py b/lib/matplotlib/axes/_axes.py
index c14a01fa3a98..89fc3ba576d6 100644
--- a/lib/matplotlib/axes/_axes.py
+++ b/lib/matplotlib/axes/_axes.py
@@ -2163,12 +2163,12 @@ def _convert_dx(dx, x0, xconv, convert):
             # removes the units from unit packages like `pint` that
             # wrap numpy arrays.
             try:
-                x0 = x0[0]
+                x0 = cbook.safe_first_element(x0)
             except (TypeError, IndexError, KeyError):
                 x0 = x0
 
             try:
-                x = xconv[0]
+                x = cbook.safe_first_element(xconv)
             except (TypeError, IndexError, KeyError):
                 x = xconv
 
diff --git a/lib/matplotlib/axes/_base.py b/lib/matplotlib/axes/_base.py
index ff5aaf032f51..c2a4d2f42c81 100644
--- a/lib/matplotlib/axes/_base.py
+++ b/lib/matplotlib/axes/_base.py
@@ -3267,7 +3267,8 @@ def set_xlim(self, left=None, right=None, emit=True, auto=False,
         reverse = left > right
         left, right = self.xaxis.get_major_locator().nonsingular(left, right)
         left, right = self.xaxis.limit_range_for_scale(left, right)
-        left, right = sorted([left, right], reverse=reverse)
+        # cast to bool to avoid bad interaction between python 3.8 and np.bool_
+        left, right = sorted([left, right], reverse=bool(reverse))
 
         self.viewLim.intervalx = (left, right)
         if auto is not None:
@@ -3649,7 +3650,8 @@ def set_ylim(self, bottom=None, top=None, emit=True, auto=False,
         reverse = bottom > top
         bottom, top = self.yaxis.get_major_locator().nonsingular(bottom, top)
         bottom, top = self.yaxis.limit_range_for_scale(bottom, top)
-        bottom, top = sorted([bottom, top], reverse=reverse)
+        # cast to bool to avoid bad interaction between python 3.8 and np.bool_
+        bottom, top = sorted([bottom, top], reverse=bool(reverse))
 
         self.viewLim.intervaly = (bottom, top)
         if auto is not None:
diff --git a/lib/matplotlib/axis.py b/lib/matplotlib/axis.py
index a4cdffc55e14..e23b5748cf7c 100644
--- a/lib/matplotlib/axis.py
+++ b/lib/matplotlib/axis.py
@@ -2156,7 +2156,8 @@ def get_minpos(self):
     def set_inverted(self, inverted):
         # docstring inherited
         a, b = self.get_view_interval()
-        self.axes.set_xlim(sorted((a, b), reverse=inverted), auto=None)
+        # cast to bool to avoid bad interaction between python 3.8 and np.bool_
+        self.axes.set_xlim(sorted((a, b), reverse=bool(inverted)), auto=None)
 
     def set_default_intervals(self):
         # docstring inherited
@@ -2463,7 +2464,8 @@ def get_minpos(self):
     def set_inverted(self, inverted):
         # docstring inherited
         a, b = self.get_view_interval()
-        self.axes.set_ylim(sorted((a, b), reverse=inverted), auto=None)
+        # cast to bool to avoid bad interaction between python 3.8 and np.bool_
+        self.axes.set_ylim(sorted((a, b), reverse=bool(inverted)), auto=None)
 
     def set_default_intervals(self):
         # docstring inherited
diff --git a/lib/matplotlib/backend_bases.py b/lib/matplotlib/backend_bases.py
index 08357ab0b99a..0be54ae14fd7 100644
--- a/lib/matplotlib/backend_bases.py
+++ b/lib/matplotlib/backend_bases.py
@@ -1532,6 +1532,20 @@ def __init__(self, name, canvas, key, x=0, y=0, guiEvent=None):
         self.key = key
 
 
+def _is_non_interactive_terminal_ipython(ip):
+    """
+    Return whether we are in a a terminal IPython, but non interactive.
+
+    When in _terminal_ IPython, ip.parent will have and `interact` attribute,
+    if this attribute is False we do not setup eventloop integration as the
+    user will _not_ interact with IPython. In all other case (ZMQKernel, or is
+    interactive), we do.
+    """
+    return (hasattr(ip, 'parent')
+        and (ip.parent is not None)
+        and getattr(ip.parent, 'interact', None) is False)
+
+
 class FigureCanvasBase(object):
     """
     The canvas the figure renders into.
@@ -1620,15 +1634,8 @@ def _fix_ipython_backend2gui(cls):
         backend2gui_rif = {"qt5": "qt", "qt4": "qt", "gtk3": "gtk3",
                            "wx": "wx", "macosx": "osx"}.get(rif)
         if backend2gui_rif:
-            pt.backend2gui[get_backend()] = backend2gui_rif
-            # Work around pylabtools.find_gui_and_backend always reading from
-            # rcParamsOrig.
-            orig_origbackend = mpl.rcParamsOrig["backend"]
-            try:
-                mpl.rcParamsOrig["backend"] = mpl.rcParams["backend"]
-                ip.enable_matplotlib()
-            finally:
-                mpl.rcParamsOrig["backend"] = orig_origbackend
+            if _is_non_interactive_terminal_ipython(ip):
+                ip.enable_gui(backend2gui_rif)
 
     @contextmanager
     def _idle_draw_cntx(self):
diff --git a/lib/matplotlib/backends/backend_qt5.py b/lib/matplotlib/backends/backend_qt5.py
index 1e60ec14dab9..d7a4fd0f0ca0 100644
--- a/lib/matplotlib/backends/backend_qt5.py
+++ b/lib/matplotlib/backends/backend_qt5.py
@@ -492,7 +492,8 @@ def draw_idle(self):
         # current event loop in order to ensure thread affinity and to
         # accumulate multiple draw requests from event handling.
         # TODO: queued signal connection might be safer than singleShot
-        if not (self._draw_pending or self._is_drawing):
+        if not (getattr(self, '_draw_pending', False) or
+                getattr(self, '._is_drawing', False)):
             self._draw_pending = True
             QtCore.QTimer.singleShot(0, self._draw_idle)
 
diff --git a/lib/matplotlib/backends/backend_qt5agg.py b/lib/matplotlib/backends/backend_qt5agg.py
index 4dec3b9cb0c4..f1fc3555863d 100644
--- a/lib/matplotlib/backends/backend_qt5agg.py
+++ b/lib/matplotlib/backends/backend_qt5agg.py
@@ -38,16 +38,21 @@ def paintEvent(self, event):
 
         painter = QtGui.QPainter(self)
 
-        rect = event.rect()
-        left = rect.left()
-        top = rect.top()
-        width = rect.width()
-        height = rect.height()
         # See documentation of QRect: bottom() and right() are off by 1, so use
         # left() + width() and top() + height().
-        bbox = Bbox(
-            [[left, self.renderer.height - (top + height * self._dpi_ratio)],
-             [left + width * self._dpi_ratio, self.renderer.height - top]])
+        rect = event.rect()
+        # scale rect dimensions using the screen dpi ratio to get
+        # correct values for the Figure coordinates (rather than QT5's coords)
+        width = rect.width() * self._dpi_ratio
+        height = rect.height() * self._dpi_ratio
+        left, top = self.mouseEventCoords(rect.topLeft())
+        # shift the "top" by the height of the image to get the
+        # correct corner for our coordinate system
+        bottom = top - height
+        # same with the right side of the image
+        right = left + width
+        # create a buffer using the image bounding box
+        bbox = Bbox([[left, bottom], [right, top]])
         reg = self.copy_from_bbox(bbox)
         buf = cbook._unmultiplied_rgba8888_to_premultiplied_argb32(
             memoryview(reg))
@@ -60,8 +65,9 @@ def paintEvent(self, event):
         if hasattr(qimage, 'setDevicePixelRatio'):
             # Not available on Qt4 or some older Qt5.
             qimage.setDevicePixelRatio(self._dpi_ratio)
-        origin = QtCore.QPoint(left, top)
-        painter.drawImage(origin / self._dpi_ratio, qimage)
+        # set origin using original QT coordinates
+        origin = QtCore.QPoint(rect.left(), rect.top())
+        painter.drawImage(origin, qimage)
         # Adjust the buf reference count to work around a memory
         # leak bug in QImage under PySide on Python 3.
         if QT_API in ('PySide', 'PySide2'):
diff --git a/lib/matplotlib/dates.py b/lib/matplotlib/dates.py
index 663d280b0347..54f903ddc6b3 100644
--- a/lib/matplotlib/dates.py
+++ b/lib/matplotlib/dates.py
@@ -836,7 +836,7 @@ def __init__(self, locator, tz=None, formats=None, offset_formats=None,
         # like 1 Jan can just be labled "Jan".  02:02:00 can
         # just be labeled 02:02.
         if zero_formats:
-            if len(formats) != 6:
+            if len(zero_formats) != 6:
                 raise ValueError('zero_formats argument must be a list of '
                                  '6 format strings (or None)')
             self.zero_formats = zero_formats
diff --git a/lib/matplotlib/pyplot.py b/lib/matplotlib/pyplot.py
index cadb906ab065..782b6897204f 100644
--- a/lib/matplotlib/pyplot.py
+++ b/lib/matplotlib/pyplot.py
@@ -790,7 +790,7 @@ def axes(arg=None, **kwargs):
 
     Parameters
     ----------
-    arg : { None, 4-tuple, Axes }
+    arg : None or 4-tuple
         The exact behavior of this function depends on the type:
 
         - *None*: A new full window axes is added using
@@ -798,13 +798,6 @@ def axes(arg=None, **kwargs):
         - 4-tuple of floats *rect* = ``[left, bottom, width, height]``.
           A new axes is added with dimensions *rect* in normalized
           (0, 1) units using `~.Figure.add_axes` on the current figure.
-        - `~.axes.Axes`: This is equivalent to `.pyplot.sca`.
-          It sets the current axes to *arg*. Note: This implicitly
-          changes the current figure to the parent of *arg*.
-
-          .. note:: The use of an `.axes.Axes` as an argument is deprecated
-                    and will be removed in v3.0. Please use `.pyplot.sca`
-                    instead.
 
     projection : {None, 'aitoff', 'hammer', 'lambert', 'mollweide', \
 'polar', 'rectilinear', str}, optional
diff --git a/lib/matplotlib/rcsetup.py b/lib/matplotlib/rcsetup.py
index 17d5d7a6dc47..31f755e4375a 100644
--- a/lib/matplotlib/rcsetup.py
+++ b/lib/matplotlib/rcsetup.py
@@ -461,6 +461,19 @@ def validate_fontsize(s):
 validate_fontsizelist = _listify_validator(validate_fontsize)
 
 
+def validate_fontweight(s):
+    weights = [
+        'ultralight', 'light', 'normal', 'regular', 'book', 'medium', 'roman',
+        'semibold', 'demibold', 'demi', 'bold', 'heavy', 'extra bold', 'black']
+    # Note: Historically, weights have been case-sensitive in Matplotlib
+    if s in weights:
+        return s
+    try:
+        return int(s)
+    except (ValueError, TypeError):
+        raise ValueError(f'{s} is not a valid font weight. %s')
+
+
 def validate_font_properties(s):
     parse_fontconfig_pattern(s)
     return s
@@ -1113,7 +1126,7 @@ def _validate_linestyle(ls):
     'font.style':      ['normal', validate_string],
     'font.variant':    ['normal', validate_string],
     'font.stretch':    ['normal', validate_string],
-    'font.weight':     ['normal', validate_string],
+    'font.weight':     ['normal', validate_fontweight],
     'font.size':       [10, validate_float],      # Base font size in points
     'font.serif':      [['DejaVu Serif', 'Bitstream Vera Serif',
                          'Computer Modern Roman',
@@ -1190,7 +1203,7 @@ def _validate_linestyle(ls):
 
     'axes.titlesize':        ['large', validate_fontsize],  # fontsize of the
                                                             # axes title
-    'axes.titleweight':      ['normal', validate_string],  # font weight of axes title
+    'axes.titleweight':      ['normal', validate_fontweight],  # font weight of axes title
     'axes.titlepad':         [6.0, validate_float],  # pad from axes top to title in points
     'axes.grid':             [False, validate_bool],   # display grid or not
     'axes.grid.which':       ['major', validate_axis_locator],  # set whether the gid are by
@@ -1202,7 +1215,7 @@ def _validate_linestyle(ls):
     'axes.labelsize':        ['medium', validate_fontsize],  # fontsize of the
                                                              # x any y labels
     'axes.labelpad':         [4.0, validate_float],  # space between label and axis
-    'axes.labelweight':      ['normal', validate_string],  # fontsize of the x any y labels
+    'axes.labelweight':      ['normal', validate_fontweight],  # fontsize of the x any y labels
     'axes.labelcolor':       ['black', validate_color],    # color of axis label
     'axes.formatter.limits': [[-7, 7], validate_nseq_int(2)],
                                # use scientific notation if log10
@@ -1340,7 +1353,7 @@ def _validate_linestyle(ls):
     ## figure props
     # figure title
     'figure.titlesize':   ['large', validate_fontsize],
-    'figure.titleweight': ['normal', validate_string],
+    'figure.titleweight': ['normal', validate_fontweight],
 
     # figure size in inches: width by height
     'figure.figsize':    [[6.4, 4.8], validate_nseq_float(2)],
diff --git a/lib/matplotlib/sphinxext/plot_directive.py b/lib/matplotlib/sphinxext/plot_directive.py
index 8d9996c13830..5ca7b8e66d30 100644
--- a/lib/matplotlib/sphinxext/plot_directive.py
+++ b/lib/matplotlib/sphinxext/plot_directive.py
@@ -367,7 +367,7 @@ def remove_coding(text):
 TEMPLATE = """
 {{ source_code }}
 
-{{ only_html }}
+.. only:: html
 
    {% if source_link or (html_show_formats and not multi_image) %}
    (
@@ -403,27 +403,15 @@ def remove_coding(text):
       {{ caption }}
    {% endfor %}
 
-{{ only_latex }}
+.. only:: not html
 
    {% for img in images %}
-   {% if 'pdf' in img.formats -%}
-   .. figure:: {{ build_dir }}/{{ img.basename }}.pdf
+   .. figure:: {{ build_dir }}/{{ img.basename }}.*
       {% for option in options -%}
       {{ option }}
       {% endfor %}
 
       {{ caption }}
-   {% endif -%}
-   {% endfor %}
-
-{{ only_texinfo }}
-
-   {% for img in images %}
-   .. image:: {{ build_dir }}/{{ img.basename }}.png
-      {% for option in options -%}
-      {{ option }}
-      {% endfor %}
-
    {% endfor %}
 
 """
@@ -794,10 +782,6 @@ def run(arguments, content, options, state_machine, state, lineno):
             ':%s: %s' % (key, val) for key, val in options.items()
             if key in ('alt', 'height', 'width', 'scale', 'align', 'class')]
 
-        only_html = ".. only:: html"
-        only_latex = ".. only:: latex"
-        only_texinfo = ".. only:: texinfo"
-
         # Not-None src_link signals the need for a source link in the generated
         # html
         if j == 0 and config.plot_html_show_source_link:
@@ -811,9 +795,6 @@ def run(arguments, content, options, state_machine, state, lineno):
             build_dir=build_dir_link,
             source_link=src_link,
             multi_image=len(images) > 1,
-            only_html=only_html,
-            only_latex=only_latex,
-            only_texinfo=only_texinfo,
             options=opts,
             images=images,
             source_code=source_code,
diff --git a/lib/matplotlib/tests/test_axes.py b/lib/matplotlib/tests/test_axes.py
index 5cdabee201f7..8ff5ac45d27d 100644
--- a/lib/matplotlib/tests/test_axes.py
+++ b/lib/matplotlib/tests/test_axes.py
@@ -1592,6 +1592,14 @@ def test_bar_pandas(pd):
     ax.plot(dates, baseline, color='orange', lw=4)
 
 
+def test_bar_pandas_indexed(pd):
+    # Smoke test for indexed pandas
+    df = pd.DataFrame({"x": [1., 2., 3.], "width": [.2, .4, .6]},
+                       index=[1, 2, 3])
+    fig, ax = plt.subplots()
+    ax.bar(df.x, 1., width=df.width)
+
+
 @image_comparison(baseline_images=['hist_log'],
                   remove_text=True)
 def test_hist_log():
diff --git a/lib/matplotlib/tests/test_backend_qt.py b/lib/matplotlib/tests/test_backend_qt.py
index f6493cfb9c38..0d4f1bec56a6 100644
--- a/lib/matplotlib/tests/test_backend_qt.py
+++ b/lib/matplotlib/tests/test_backend_qt.py
@@ -307,3 +307,23 @@ def test_figureoptions():
             "matplotlib.backends.qt_editor._formlayout.FormDialog.exec_",
             lambda self: None):
         fig.canvas.manager.toolbar.edit_parameters()
+
+
+@pytest.mark.backend("Qt5Agg")
+def test_canvas_reinit():
+    import matplotlib.pyplot as plt
+    from matplotlib.backends.backend_qt5agg import FigureCanvasQTAgg
+    from functools import partial
+
+    called = False
+
+    def crashing_callback(fig, stale):
+        nonlocal called
+        fig.canvas.draw_idle()
+        called = True
+
+    fig, ax = plt.subplots()
+    fig.stale_callback = crashing_callback
+    # this should not raise
+    canvas = FigureCanvasQTAgg(fig)
+    assert called
diff --git a/lib/matplotlib/tests/test_constrainedlayout.py b/lib/matplotlib/tests/test_constrainedlayout.py
index df3e5cf18def..8769fcec3537 100644
--- a/lib/matplotlib/tests/test_constrainedlayout.py
+++ b/lib/matplotlib/tests/test_constrainedlayout.py
@@ -404,3 +404,16 @@ def test_colorbar_location():
     fig.colorbar(pcm, ax=axs[-2, 3:], shrink=0.5, location='top')
     fig.colorbar(pcm, ax=axs[0, 0], shrink=0.5, location='left')
     fig.colorbar(pcm, ax=axs[1:3, 2], shrink=0.5, location='right')
+
+
+def test_hidden_axes():
+    # test that if we make an axes not visible that constrained_layout
+    # still works.  Note the axes still takes space in the layout
+    # (as does a gridspec slot that is empty)
+    fig, axs = plt.subplots(2, 2, constrained_layout=True)
+    axs[0, 1].set_visible(False)
+    fig.canvas.draw()
+    extents1 = np.copy(axs[0, 0].get_position().extents)
+
+    np.testing.assert_allclose(extents1,
+        [0.045552, 0.548288, 0.47319, 0.982638], rtol=1e-5)
diff --git a/lib/matplotlib/tests/test_image.py b/lib/matplotlib/tests/test_image.py
index ae803000ddee..22cd78d46820 100644
--- a/lib/matplotlib/tests/test_image.py
+++ b/lib/matplotlib/tests/test_image.py
@@ -989,3 +989,18 @@ def test_deprecation():
         with pytest.warns(MatplotlibDeprecationWarning):
             # Enough arguments to pass "shape" positionally.
             obj.imshow(data, *[None] * 10)
+
+
+def test_image_cursor_formatting():
+    fig, ax = plt.subplots()
+    # Create a dummy image to be able to call format_cursor_data
+    im = ax.imshow(np.zeros((4, 4)))
+
+    data = np.ma.masked_array([0], mask=[True])
+    assert im.format_cursor_data(data) == '[]'
+
+    data = np.ma.masked_array([0], mask=[False])
+    assert im.format_cursor_data(data) == '[0]'
+
+    data = np.nan
+    assert im.format_cursor_data(data) == '[nan]'
diff --git a/lib/matplotlib/tests/test_rcparams.py b/lib/matplotlib/tests/test_rcparams.py
index 123ffe4ae3c3..d8fe73cebc2a 100644
--- a/lib/matplotlib/tests/test_rcparams.py
+++ b/lib/matplotlib/tests/test_rcparams.py
@@ -17,6 +17,7 @@
                                 validate_colorlist,
                                 validate_color,
                                 validate_bool,
+                                validate_fontweight,
                                 validate_nseq_int,
                                 validate_nseq_float,
                                 validate_cycler,
@@ -412,6 +413,26 @@ def test_validator_invalid(validator, arg, exception_type):
         validator(arg)
 
 
+@pytest.mark.parametrize('weight, parsed_weight', [
+    ('bold', 'bold'),
+    ('BOLD', ValueError),  # weight is case-sensitive
+    (100, 100),
+    ('100', 100),
+    (np.array(100), 100),
+    # fractional fontweights are not defined. This should actually raise a
+    # ValueError, but historically did not.
+    (20.6, 20),
+    ('20.6', ValueError),
+    ([100], ValueError),
+])
+def test_validate_fontweight(weight, parsed_weight):
+    if parsed_weight is ValueError:
+        with pytest.raises(ValueError):
+            validate_fontweight(weight)
+    else:
+        assert validate_fontweight(weight) == parsed_weight
+
+
 def test_keymaps():
     key_list = [k for k in mpl.rcParams if 'keymap' in k]
     for k in key_list:
diff --git a/lib/matplotlib/ticker.py b/lib/matplotlib/ticker.py
index 3f6130164814..d1aa0c7fd112 100644
--- a/lib/matplotlib/ticker.py
+++ b/lib/matplotlib/ticker.py
@@ -181,10 +181,11 @@
            'LogFormatterExponent', 'LogFormatterMathtext',
            'IndexFormatter', 'LogFormatterSciNotation',
            'LogitFormatter', 'EngFormatter', 'PercentFormatter',
+           'OldScalarFormatter',
            'Locator', 'IndexLocator', 'FixedLocator', 'NullLocator',
            'LinearLocator', 'LogLocator', 'AutoLocator',
            'MultipleLocator', 'MaxNLocator', 'AutoMinorLocator',
-           'SymmetricalLogLocator', 'LogitLocator')
+           'SymmetricalLogLocator', 'LogitLocator', 'OldAutoLocator')
 
 
 def _mathdefault(s):
@@ -628,6 +629,8 @@ def format_data_short(self, value):
         """
         if self._useLocale:
             return locale.format_string('%-12g', (value,))
+        elif isinstance(value, np.ma.MaskedArray) and value.mask:
+            return ''
         else:
             return '%-12g' % value
 
diff --git a/lib/mpl_toolkits/mplot3d/axes3d.py b/lib/mpl_toolkits/mplot3d/axes3d.py
index 4202be9a1138..047234b365f5 100644
--- a/lib/mpl_toolkits/mplot3d/axes3d.py
+++ b/lib/mpl_toolkits/mplot3d/axes3d.py
@@ -626,7 +626,8 @@ def set_xlim3d(self, left=None, right=None, emit=True, auto=False,
         reverse = left > right
         left, right = self.xaxis.get_major_locator().nonsingular(left, right)
         left, right = self.xaxis.limit_range_for_scale(left, right)
-        left, right = sorted([left, right], reverse=reverse)
+        # cast to bool to avoid bad interaction between python 3.8 and np.bool_
+        left, right = sorted([left, right], reverse=bool(reverse))
         self.xy_viewLim.intervalx = (left, right)
 
         if auto is not None: