The image-button style class takes care of it.
35f4243 example: Make search button square
examples/application10/window.ui | 3 +++
examples/application7/window.ui | 3 +++
examples/application8/window.ui | 3 +++
examples/application9/window.ui | 3 +++
4 files changed, 12 insertions(+)
Upstream: git.gnome.org