Get theme transition duration for scroll bar

Question

I am trying to implement a custom control which, as part of its WM_PAINT handler, paints a themed scroll bar arrow. I also want transitions between the visual states of the button (normal, hot, hover, pressed, and so forth) to have a fade animation using the BeginBufferedAnimation() family of functions defined in uxtheme.dll.

My problem right now is that I can't get the correct fade duration for scrollbar because the GetThemeTransitionDuration() function is failing. Instead of succeeding, it returns E_INVALIDARG (-2147024809) when I attempt to query the transition duration:

HTHEME hTheme = OpenThemeData(hwnd, L"SCROLLBAR"); // returns a valid non-NULL theme handle
...
DWORD dwDuration = 0;
HRESULT hr = GetThemeTransitionDuration(
    hTheme,                /*hTheme*/
    SBP_ARROWBTN           /*iPartId*/,
    ABP_LEFTNORMAL         /*iStateIdFrom*/,
    ABP_LEFTHOT            /*iStateIdTo*/,
    TMT_TRANSITIONDURATION /*iPropId*/, 
    &dwDuration            /*pdwDuration*/
); // PROBLEM: returns E_INVALIDARG

This is weird for two reasons:

(1) In my version of Windows 7, the themed scrollbars clearly have quick fade animations between states (e.g. in Windows Explorer).

(2) If I change the theme to BUTTON and just use the normal and hot button states, the function returns S_OK and gives a reasonable transition duration (1000ms on my machine).

HTHEME hTheme = OpenThemeData(hwnd, L"BUTTON");
...
DWORD dwDuration = 0;
HRESULT hr = GetThemeTransitionDuration(
    hTheme,                /*hTheme*/
    BP_PUSHBUTTON          /*iPartId*/,
    PBS_NORMAL             /*iStateIdFrom*/,
    PBS_HOT                /*iStateIdTo*/,
    TMT_TRANSITIONDURATION /*iPropId*/, 
    &dwDuration            /*pdwDuration*/
); // returns S_OK, and dwDuration = 1000

Is there a way to query the transition duration I'm looking for?