Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?

go version go1.6.2 darwin/amd64

1. What operating system and processor architecture are you using (go env)?
   GOARCH="amd64"
   GOBIN=""
   GOEXE=""
   GOHOSTARCH="amd64"
   GOHOSTOS="darwin"
   GOOS="darwin"
   GOPATH="/Users/driusan/Code/Go"
   GORACE=""
   GOROOT="/usr/local/go"
   GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
   GO15VENDOREXPERIMENT="1"
   CC="clang"
   GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fno-common"
   CXX="clang++"
   CGO_ENABLED="1"
2. What did you do?
   If possible, provide a recipe for reproducing the error.
   A complete runnable program is good.
   A link on play.golang.org is best.

Ran x/exp/shiny/example/basic, scrolled mouse a single click with the scroll wheel on a mouse

1. What did you expect to see?

Output similar to:

got mouse.Event{X:824, Y:452, Button:-1, Modifiers:0x0, Direction:0x0}

in the console.

1. What did you see instead?

Nothing.

Adding some debug statements, it seems that the issue is that this line:

for delta := int(dy); delta != 0; delta--

in driver/gldriver/cocoa.go:/func mouseEvent/ isn't iterating at all when dy < 1 and the conversion is rounding to 0 when it casts to an int.

Changing it to:
for delta := int(math.Ceil(float64(dy))); delta != 0; delta--

if a little inelegant, fixed the issue for me.