# Notebook with a WindowOpacity too small becomes “click-through” for mouse

I created a notebook and was manipulating its WindowOpacity option:

nb = CreateDocument[];
Manipulate[SetOptions[nb, WindowOpacity -> op], {{op, .3}, 0, 1}]


It works fine as a half-transparent window when the option has a value not too small, say 0.3 (Have written something in nb to make it more prominent against the web brower window behind):

But when the value of the option is less than a certain threshold (in my case 0.004), the nb window seems to be "click-through" for mouse, eventhough it should be still in place in front of the web brower window:

The question: Is this "click-through" behavior a feature or a bug-like thing? And why such a nonzero threshold?

ps. My environment is Windows 7 (x64) and Mathematica 8.0.4.

-
Behavior confirmed in version 7 on Windows 7. –  Mr.Wizard Aug 12 '12 at 10:28
Same in version 8 on XP Pro SP3. –  stevenvh Aug 12 '12 at 10:35
Confirmed on Mac OSX Lion, V 8.0.4. –  JxB Aug 12 '12 at 14:57
Might this actually be a safety feature? If you could activate something by pressing a near invisible button while the user thinks he's pressing another button that would be a kind of an exploit. –  Sjoerd C. de Vries Aug 12 '12 at 20:18
In v8 on Win7-64 the threshold is 1/255. At 1/256 the click-through occurs. I assume window opacity is stored in an 8 bit alpha channel and therefore any opacity less than 1/255 corresponds to perfect transparency. As @SjoerdC.deVries says, one wouldn't expect a transparent window to intercept clicks for security reasons. –  Simon Woods Aug 12 '12 at 21:17

nb = CreateDocument[];

At any WindowOpacity less than 1/255 the click-through occurs. It seems likely that the window opacity is stored in an 8 bit alpha channel and therefore any value less than 1/255 corresponds to perfect transparency. Presumably the click-through is the default behaviour for fully transparent windows.