See https://github.com/matplotlib/matplotlib/pull/14587

commit 564edad96232e028294d7421b41955029e16816f
Author: Jeroen Demeyer <J.Demeyer@UGent.be>
Date:   Thu Jun 20 13:45:09 2019 +0200

    Install verdored copy of jquery-ui

diff --git a/setupext.py b/setupext.py
index c75725b..3536cf6 100644
--- a/setupext.py
+++ b/setupext.py
@@ -871,6 +871,9 @@ class Matplotlib(SetupPackage):
                 'backends/web_backend/jquery/js/*.min.js',
                 'backends/web_backend/jquery/css/themes/base/*.min.css',
                 'backends/web_backend/jquery/css/themes/base/images/*',
+                'backends/web_backend/jquery-ui-*/*',
+                'backends/web_backend/jquery-ui-*/*/*',
+                'backends/web_backend/jquery-ui-*/*/*/*',
                 'backends/web_backend/css/*.*',
                 'backends/Matplotlib.nib/*',
                 'mpl-data/stylelib/*.mplstyle',
