Please use setWraparoundModeHorizontalEnabled() naming convention for setters. We don't use ruby-style naming :)

I guess you just followed example of enablePixelGrid(), but that one should also be converted to the standard naming (we seem to have missed that when reviewing that patch).

space is lost :)

Oh... this change is a bit dangerous in a long-run. One day someone will try to use the calculated values without calling updateActiveCanvasArea() in advance and will break everything :)

I guess you tried to solved two problems in the same time: return two values from calculation function and (in perspective) optimize this calculation with caching. Such situations are quite common, and there are a few rules of thumb for them:

1. Do not store calculated temporary values anywhere outside minimally necessary scope. The values should be discarded right after they have been used.

2. If the values are still need to be stored for caching reasons, ensure either that the values are in always valid state or there is an explicit mark that they are dirty. In the case of activeCanvasArea and canvasWidgetRect, they should be automatically regenerated (or marked as dirty) every time the canvas widget is resized or image is resized/panned/zoomed/rotated. That is quite a lot of (unnecessary) work.

If these rules are not followed, some other developer in the future will try to use these values in expectation that they are "always valid" and will break everything :)

To solve the problem of returning multiple values from a function in C++, you can use either of the two methods:

Pass by pointer (the simplest solution):

void KisOpenGLCanvas2::calculateActiveCanvasArea(QRect *activeCanvasArea, QRect *canvasWidgetRect) const;

Or return a structure (usually it is needed for undo states or something that has multiple instances):

struct CanvasAreaState {
    QRect activeCanvasArea;
    QRect canvasWidgetRect;
}; 
CanvasAreaState KisOpenGLCanvas2::calculateActiveCanvasArea() const;

I would prefer the first method, but it is mostly a matter of taste :)

PS:
And, yes, without storing the temporaries the calculation method becomes 'const', which is exactly what it is semantically.