This is a minimal reproducer for a failure in the pipeline #29125 (comment).

The test_interactive_backend[toolmanager-MPLBACKEND=wxagg-BACKEND_DEPS=wx] fails in the pipeline at the subsequent assert result.getvalue() == result_after.getvalue() because the PNG data in result and result_after have different sizes (640x480) vs. (640x478); i.e. result_after is two pixels smaller. This stems from a changed fig size, as made explicit by the assert statement added here (precisely, fig.get_size_inches() changes from np.array([6.4, 4.8]) to np.array([6.4, 4.78])).

I have not yet a clue why closing the figure window could change the figure size. Actually, showing the window changes the size, see below.

Note: reading the exception is a bit complicated because it happens in a called subprocess, but you'll find this somewhere in the middle of the traceback:

[...] assert tuple(fig.get_size_inches()) == tuple(size_inches), \\\nAssertionError: [6.4  4.78], [6.4 4.8]\n'

Update: This is the call stack that leads to the changed figure size:

  File "/home/tim/git/matplotlib/lib/matplotlib/tests/test_backends_interactive.py", line 223, in _test_interactive_impl
    plt.show()
  File "/home/tim/git/matplotlib/lib/matplotlib/pyplot.py", line 612, in show
    return _get_backend_mod().show(*args, **kwargs)
  File "/home/tim/git/matplotlib/lib/matplotlib/backend_bases.py", line 3534, in show
    manager.show()  # Emits a warning for non-interactive backend.
  File "/home/tim/git/matplotlib/lib/matplotlib/backends/backend_wx.py", line 1006, in show
    self.frame.Show()
  File "/home/tim/git/matplotlib/lib/matplotlib/backends/backend_wx.py", line 683, in _on_size
    self.figure.set_size_inches(winch, hinch, forward=False)

Apparently show() creates a wx.EVT_SIZE, which is bound to _FigureCanvasWxBase._on_size(), and in there

returns the reduced height of 478.