Why does my VCL form created inside a DLL disappear when minimised

Question

I have a Windows VCL application written in C++Builder. Some of the UI is implemented inside a DLL rather than directly in the main EXE.

The DLL exports a function that creates and shows a VCL form, something like:

extern "C" __declspec(dllexport)
void __stdcall show_plugin_form()
{
  auto *f(new TPluginForm(nullptr));
  f->Show();
}

Owner is intentionally nullptr because the form is created inside a DLL and I don't want it auto-destroyed by the EXE's main form.

The form displays correctly and behaves normally until I minimise it. As soon as it is minimised:

the form does not appear on the taskbar;
it also doesn't appear in the Alt+Tab list.

It seems to vanish, even though the process is still running. It becomes impossible to restore unless I explicitly call f->Show().

I checked common properties:

FormStyle = fsNormal;
BorderIcons includes biSystemMenu and biMinimize;
BorderStyle is sizeable;
WindowState = wsNormal.

manlio · Accepted Answer · 2025-11-19 08:35:32Z

1

This is expected behaviour for ownerless forms created in DLLs (a VCL form inside a DLL has no real taskbar owner).

Starting from C++Builder Athens (RAD Studio 12) the fix, based on the ShowInTaskBar property, is simple, add:

ShowInTaskBar = true;

in the form constructor.

Older versions of C++Builder have no ShowInTaskBar property, so you have to use the Windows API directly:

{
  // ...
  auto exStyle(GetWindowLong(Handle, GWL_EXSTYLE));
  exStyle |= WS_EX_APPWINDOW;  // force taskbar visibility
  SetWindowLong(Handle, GWL_EXSTYLE, exStyle);
}

answered Nov 19 at 8:35

manlio

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