... tools' brush options.
After discussions, it turned out that many people disliked that the spin
scale for brush size (and some other options) get you fractional values.
How often do you actually need to get a 4.32 pixel-size brush? And even
how meaningful is it? On the other hand, you usually want a 4 or a 5
pixel size brush and it's nearly impossible to get (exactly) by dragging
the scale widget.
It is so annoying that some even resort to edit the value with keyboard!
So I am adding an optional "constrain" feature to GimpSpinScale. It will
still be possible to get fractional values when constraining is on, for
instance with keyboard edit (the arrow incrementation also will keep any
fractional part). So the interaction for such scales is simply reversed
so that you get integers easily, and fractional parts with a bit more
effort.

It is not turned on by default (some feature actually need precision and
we don't want to break the sliders for these) and for the time being, I
only applied it to all the brush settings in paint tools. Now that it
exist, we may want to apply this to more scales in various parts of
GIMP.