diff -r f747bb29e454 indra/newview/llmaniprotate.cpp
--- a/indra/newview/llmaniprotate.cpp   Mon Aug 18 14:36:17 2014 -0400
+++ b/indra/newview/llmaniprotate.cpp   Wed Aug 20 19:29:53 2014 +0100
@@ -1239,12 +1239,12 @@

        F32 dist_from_sphere_center = sqrt(delta_x * delta_x + delta_y * delta_y);

-       LLVector3 axis = mMouseDown % mMouseCur;
-       axis.normVec();
-       F32 angle = acos(mMouseDown * mMouseCur);
-       LLQuaternion sphere_rot( angle, axis );
-
-       if (is_approx_zero(1.f - mMouseDown * mMouseCur))
+    LLVector3 axis = mMouseDown % mMouseCur;
+    axis.normVec();
+    F32 angle = atan2(sqrtf(axis * axis), mMouseDown * mMouseCur);
+    LLQuaternion sphere_rot( angle, axis );
+
+    if (is_approx_zero(1.f - mMouseDown * mMouseCur))
        {
                return LLQuaternion::DEFAULT;
        }
@@ -1638,9 +1638,9 @@
                        mInSnapRegime = FALSE;
                }

-               angle = acos(mMouseCur * mMouseDown);
-
-               F32 dir = (mMouseDown % mMouseCur) * constraint_axis;  // cross product
+        LLVector3 cross_product = mMouseDown % mMouseCur;
+        angle = atan2(sqrtf(cross_product * cross_product), mMouseCur * mMouseDown);
+        F32 dir = cross_product * constraint_axis;  // cross product
                if( dir < 0.f )
                {
                        angle *= -1.f;