How can I set SelStart and SelLength when setting focus to a TComboBox

When I move focus with the Tab key to a VCL TComboBox control containing text in the Text field, the complete text is always displayed as selected.

How can this be changed so that the selection can be customized in C++Builder?

Changing SelStart and SelLength in OnEnter doesn't seem to help.

What am I doing wrong?

One option is to have your OnEnter handler use TThread::ForceQueue() to delay your custom selection until after the standard selection gets set first, eg:

void __fastcall TMyForm::ComboBox1Enter(TObject *Sender)
{
    TThread::ForceQueue(NULL, &DoMySelect);
}

void __fastcall TMyForm::DoMySelect()
{
    ComboBox1->SelStart = ...;
    ComboBox1->SelLength = ...;
}

Or, if you are using one of the clang-based compilers, you can use a C++ lambda instead of a separate class method, eg:

void __fastcall TMyForm::ComboBox1Enter(TObject *Sender)
{
    TThread::ForceQueue(nullptr,
        [this](){
            ComboBox1->SelStart = ...;
            ComboBox1->SelLength = ...;
        }
    );
}