Adding stylesheet to QSpinBox causes a frame to be drawn to the value part

Adding a style sheet like this causes a frame to be drawn around the value in QSpinBox, see screenshots.

QSpinBox::up-button, QSpinBox::down-button {width: 18px;}

From customer:

"Have a look at the attached test app. This displays a QSpinBox with the
following stylesheet set:

QSpinBox::up-button, QSpinBox::down-button {width: 18px;}

Build it against Qt 5.11.2. You'll see that the spinbox gets an extra frame
around the text control (see screenshot).

Now build it against Qt 5.10.1 (a 5.10 build I happened to have around, I
suspect any of them will work). The spinbox background is white (see other
screenshot).

"Why does this matter?" I hear you cry. The answer is that we have a
custom spinbox in our app, of which the paddle width is merely one aspect of a
complex style. We noticed this spurious frame when we switched to Qt 5.11
Through trial and error I've determined that it is the width that's causing
it.

I have a feeling this regression might be caused by the recent changes to
qmacstyle_mac.mm to remove HITheme (specifically perhaps some frame-related
ones in 0c974c0? ). I haven't had chance to experiment with reverting these
changes to see if the problem goes away, however."