X11 FocusIn is not working - linux

Question

As long as I understand it, the X11 FocusIn event is triggered whenever the window comes into focused. It the the window that the keyboard input is sent to. I am having trouble triggering this event. I have made sure to give it the FocusChangeMask when creating the window. I have created a breakpoint inside my event handler where the FocusIn event is supposed to happen and it is not stopping.
I have 2 separate window, a transparent one and one non-transparent. Currently I have it so the transparent window is always on top of the non transparent window. Whenever I switch focus and then switch back to the transparent window, the non-transparent window is directly underneath. This causes other windows to be stuck in 'between' the transparent and non-transparent window.
I have noticed that whenever I focus on the non-transparent window that is underneath that triggers the FocusIn event. I can not get the transparent window to trigger the event. Does this have something to do with the window being in 32-bit color?
What am I missing?
while(!renderer->stop)
{
XNextEvent(renderer->x_display, &event);
switch(event.type)
{
case Expose:
if (event.xexpose.window == gstreamer_window)
{
XRaiseWindow(renderer->x_display, renderer->opengl_window);
}
break;
case FocusIn:
if (event.xfocus.window == renderer->opengl_window)
{
XRaiseWindow(renderer->x_display, gstreamer_window);
}
break;
case ConfigureNotify:
if (event.xconfigure.window == renderer->opengl_window)
{
XMoveWindow(renderer->x_display, gstreamer_window,
event.xconfigure.x, event.xconfigure.y - top_border_offset);
}
break;
}
}
Here is how I created the window.
XSetWindowAttributes swa;
swa.event_mask = ExposureMask | PointerMotionMask | KeyPressMask | FocusChangeMask;
swa.colormap = XCreateColormap(x_display, XDefaultRootWindow(x_display), visual, AllocNone);
swa.background_pixel = 0;
swa.border_pixel = 0;
/* Create a window */
opengl_window = XCreateWindow (
x_display, parent,
0, 0, m_plane_width, m_plane_height, 0,
depth, InputOutput,
visual, CWEventMask | CWBackPixel | CWColormap | CWBorderPixel,
&swa );

Answer 1

It appears as though I set the FocusChangeMask at the wrong place. By adding the line XSelectInput(x_display, opengl_window, FocusChangeMask)
, it now triggers the FocusIn event. It was triggering the other display because it had the mask but this one didn't.

Answer 2

It's not clear from your question whether the windows referenced in your question are top level application windows, or secondary, ancillary windows that are child windows of your top level application windows.
But in all cases, proper input focus handling requires you to inform the window manager exactly how your application expects to handle input focus.
See section 4.1.7 of the ICCCM specification for more information. Unfortunately, it's not enough just to code an event loop that handles FocusIn messages, and expect them to fall out of the sky. First, you need to tell your window manager how exactly you are going to handle input focus switching, then respond to the window manager's messages, as explained in the ICCCM specification; perhaps by explicitly sending the SetInputFocus requests for your own application windows.

Answer 3

See my post under OP's self accepted answer for background
I had the same problem and it seems if you use XSelectInput it overrides the event mask for a window, instead of adding to it. You need to either:
skip adding XSelectInput and set the event masks on the window when created
or add all of the masks for the window to the XSelectInput command
Here's my similar setup which worked before I added the XSelectInput line:
swa.event_mask = StructureNotifyMask | SubstructureNotifyMask | FocusChangeMask | ExposureMask | KeyPressMask;
wnd = XCreateWindow(disp, wndRoot, left, top, width, height, 0, vi->depth, InputOutput, vi->visual, CWColormap | CWEventMask, &swa);
XMapWindow(disp, wnd);
When I added this for capturing key input:
XSelectInput(disp, wnd, KeyPressMask | KeyReleaseMask);
everything else stopped working, which I didn't realize for a while, until now, because I was just testing keys.
I have a single window application with no controls in the client area, so I don't know what else XSelectInput does for a window that already has the masks, but I do get key up and down messages without it. In the OP's self accepted answer he just goes the other way and adds 'FocusChangeMask' to XSelectInput instead.
The documentation for a lot of X11 is, in my opinion, a lot like the standard Apple documentation where it has the affliction of never saying what the function really actually does or affects, but tells you all about everything else. (See bottom snippet here) To be fair a good portion of the Cocoa docs are pretty decent, but maybe half are not. That goes for many or most of the parameters as well.
From Linux man page which I think is the maybe verbatim the same as the Xlib manual here https://tronche.com/gui/x/xlib/
The XSelectInput function requests that the X server report the events associated with the specified event mask.
Setting the event-mask attribute of a window overrides any previous call for the same window but not for other clients.
(Below is my own perhaps incorrect rewording of the XInputSelect docs)
This can be interpreted as perhaps:
XSelectInput sets the event-mask on a window for a display for events reported to the client. Each client can set its own independent mask for events it wants to receive with the following exceptions (skipping the exceptions).
Now the critical part:
XInputSelect will override and replace for a client any previous event-mask settings for a window on a display, including any event-mask set when creating the window.
And what I wish it was written as to make sure its understood, if true, that the XInputSelect does nothing else like enable key to character interpretation and what not, because I believe the core confusion is that people from all the various examples out there get the impression that there is some other magic besides setting mask. I'm assuming there's not, and the docs speak of no other magic besides the mask.
XInputSelect sets the event-mask for a window and allows for setting an event mask after a window is created.
And my opinion is that that line should have been first.