Apologies if this appears twice - I thought I posted before, but my response doesn’t seem to be here... Odd...

On 12 Jun 2022, at 08:37, Håvard wrote:
> 
> I add the following lines to a fltk-program (testing on examples/browser-simple) and expect to see a transparent window. That do however not happen.
> 
>   Display* display;
>   char* connection_string = NULL;
>   display = XOpenDisplay (connection_string);
>   Atom atom = XInternAtom(display, "_NET_WM_WINDOW_OPACITY", False); 
>   uint opacity = 0x80000000; /* 0x0 .. 0xffffffff */
>   XChangeProperty(display, fl_xid(win), atom, XA_CARDINAL, 32,
>             PropModeReplace, (unsigned char*)&opacity, 1L);
> 
> I run under linux/X11. How to get a transparent window ?

As Manolo has said, fltk doesn’t really support transparent colours in a window, so a shaped window might be the best we can do using fltk options (there are examples in the docs and in the test folder of using shaped window.)

The other thing you can do is to alter the translucency of the entire window (not just a region of it, though). 
I have code (attached below) that illustrates this, which worked on X11, OSX and Win32 as of 2015’ish... so probably still OK!

TBH, I thought I had a nice worked example of this, but actually all I can find now is this rather feeble OSK demo, that uses the technique to make the pop-up keyboard widget semi-transparent.

So, not great - but once you have the translucent window created, you can vary its alpha dynamically at runtime, which is quite cool, if not actually all that useful!

Might be informative for your use-case, so the code is here...

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