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Summary says it all, rumors have it that breeze-gtk doesn't support gtk-3.20.x, see also: https://bugs.kde.org/show_bug.cgi?id=361066 While that's true, it may be wise to avoid confused users and bug reports, and avoid shippoing breeze-gtk at all until this is fixed.
I guess I'll go with adding this to the package description: This package is being provided as a tech-preview only, since it is not yet complete and has known incompatibilities with gtk-3.20.x. See also: https://bugs.kde.org/show_bug.cgi?id=361066 and we can keep this bug open as a reminder, until fixed
This should be fixed now, as of plasma-5.6