I could not get set_non_modal to work. Perhaps I am missing something..

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Fl::add_timeout(2.0, Update_CB, (void*)&progress);
    win.set_non_modal();
    win.show();

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I found an old thread with some info, Greg - from 16 years ago.

https://fltk.easysw.narkive.com/zcptoTmB/always-on-top

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if you can obtain hWnd, than with WinAPI function SetWindowPos() you can set

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Also,

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This was also covered in a previous thread from April:

Subject: How to make a window "Stay On Top"

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But I could not find that thread.

I found this thread - https://fltk.easysw.narkive.com/zRSNyFX3/can-i-make-a-top-most-window-by-fltk

I will copy the code in case we lose that archive.

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//===============================================================================
#include <FL/Fl.H>
#include <FL/x.H>
#include <Fl/Fl_Window.H>
#include <windows.h>

//----------------------------------------------------------------------------
void Show_Topmost(Fl_Window* pWindow)
{
HWND hWnd= fl_xid(pWindow);
ShowWindow(hWnd, SW_SHOWNORMAL);
SetWindowPos(hWnd, HWND_TOPMOST, 0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE);
}

//----------------------------------------------------------------------------
bool Is_Topmost(Fl_Window* pWindow)
{
HWND hWnd= fl_xid(pWindow);
WINDOWINFO pwi;
if (GetWindowInfo(hWnd, &pwi)) return pwi.dwExStyle & WS_EX_TOPMOST;
else return false;
}

//----------------------------------------------------------------------------
void Clear_Topmost(Fl_Window* pWindow)
{
HWND hWnd= fl_xid(pWindow);
ShowWindow(hWnd, SW_SHOWNORMAL);
SetWindowPos(hWnd, HWND_NOTOPMOST, 0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE);
}

//----------------------------------------------------------------------------
void Show_Foreground(Fl_Window* pWindow)
{
HWND hWnd= fl_xid(pWindow);
ShowWindow(hWnd, SW_SHOWNORMAL);
if (GetForegroundWindow() != hWnd)
{
DWORD fgwpid = GetWindowThreadProcessId(GetForegroundWindow(),NULL);
DWORD mypid = GetWindowThreadProcessId(hWnd, NULL);
AttachThreadInput(mypid, fgwpid, TRUE);
BringWindowToTop(hWnd);
SetForegroundWindow(hWnd);
AttachThreadInput(mypid, fgwpid, FALSE);
}
}

//----------------------------------------------------------------------------
bool Is_Foreground(Fl_Window* pWindow)
{
return GetForegroundWindow()== fl_xid(pWindow);
}

//----------------------------------------------------------------------------
void Show_Background(Fl_Window* pWindow)
{
HWND hWnd= fl_xid(pWindow);
ShowWindow(hWnd, SW_SHOWNORMAL);
SetWindowPos(hWnd, HWND_BOTTOM, 0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE);
}

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