I cannot reproduce your issue.
Can you provide a example Java program?

> IBUS_ENABLE_SYNC_MODE=1

It would cause several regressions and the setting won't be changed by default in RHEL 9 or lower.

Could not yet reproduce the problem, but user reproduced in a patched version
in the 89th try.

The modified version was changed to not enter full screen mode and run
undecorated, as it was guessed it could be somehow related to:

https://bugs.openjdk.org/browse/JDK-8013581

Also, customer did run this small program from a ssh connection:

"""
#include <stdio.h>
#include <X11/Xlib.h>
int
main(int argc, char *argv[])
{
    Display *display = XOpenDisplay(NULL);
    Window  focus;
    int revert;
    if (display == NULL) {
	fprintf(stderr, "failed to open display\n");
	return 1;
    }
    XGetInputFocus(display, &focus, &revert);
    printf("focus = ");
    if (focus == PointerRoot)
	printf("PointerRoot");
    else if (focus == None)
	printf("None");
    else
	printf("0x%x", focus);
    putchar('\n');
    printf("revert = ");
    switch (revert) {
	case RevertToParent:
	    printf("RevertToParent");
	    break;
	case RevertToPointerRoot:
	    printf("RevertToPointerRoot");
	    break;
	case RevertToNone:
	    printf("RevertToNone");
	    break;
	default:
	    printf("%d\n", revert);
	    break;
    }
    putchar('\n');
    return 0;
}
"""

and run:

$ DISPLAY=:0 ./xtest
$ DISPLAY=:0 xwininfo -all -root
$ DISPLAY=:0 xlsclients -al

The xtest output was:

focus = 0x220001f
revert = RevertToPointerRoot

and the xwininfo relevant block is:

    0xe00021 (has no name): ()  1280x1024+0+0  +0+0
        1 child:
        0x2200007 "Test": ("sun-awt-X11-XFramePeer" "Test")  1280x1024+0+0  +0+0
           2 children:
           0x220001f "FocusProxy": ("Focus-Proxy-Window" "FocusProxy")  1x1+-1+-1  +-1+-1
           0x220001c "Content window": ("sun-awt-X11-XContentWindow" "ScrTimeSet")  1280x1061+0+-37  +0+-37
     0x2200008 "ScrTimeSet": ("Test" "Test")  1x1+1+1  +1+1

Note that the focus is in the "FocusProxy" window, but the interface is
frozen.

I played a bit with the sample java code but could not figure an easy
way to have the java app shown in xlsclients output.

Still, this is the major difference I see from my tests, in xlsclients
output:

Window 0x2000001:
  Machine:  test
  Name:  <unknown type COMPOUND_TEXT (475) or format 8>
  Icon Name:  <unknown type COMPOUND_TEXT (475) or format 8>
  Command:  gnome-software
  Instance/Class:  gnome-software/Gnome-software

and that I believe might be related to the hang.

For extra data of another possible related issue, please see bz#2225954

Do you have some idea about the
"<unknown type COMPOUND_TEXT (475) or format 8>" above?

The bug reporters should not set needinfo flags.

(In reply to Paulo Andrade from comment #12)
> Could not yet reproduce the problem, but user reproduced in a patched version
> in the 89th try.

Then you may need to investigate the customer site. The issue might be caused by the wrong configurations.

> 
> The modified version was changed to not enter full screen mode and run
> undecorated, as it was guessed it could be somehow related to:
> 
> https://bugs.openjdk.org/browse/JDK-8013581

The Java issue is not my responsible and I'm not sure about it.

> The xtest output was:
> 
> focus = 0x220001f
> revert = RevertToPointerRoot

It should be normal.

> Window 0x2000001:
>   Machine:  test
>   Name:  <unknown type COMPOUND_TEXT (475) or format 8>
>   Icon Name:  <unknown type COMPOUND_TEXT (475) or format 8>
>   Command:  gnome-software
>   Instance/Class:  gnome-software/Gnome-software
> 
> and that I believe might be related to the hang.

Should not be relative with your issue.

> Do you have some idea about the
> "<unknown type COMPOUND_TEXT (475) or format 8>" above?

It should be correct. X sends several types of formats with Unicode, compound-text and  format 8.