R – Why don’t I get hotkey underlines in a Delphi TMainMenu

There is a standard Windows setting (under display properties) to normally hide those accelerators unless the Alt key is held down. That would explain why opening the menu with Alt+F10 shows them for you. Maybe that's the cause?

[EDIT]: No, it's not. I just tried, and a simple TForm with a menu item shows the accelerator, but as soon as I add a TImageList and set the ImageIndex of the single menu item, or simply set OwnerDraw to true, then the accelerator underline disappears. I guess that really is a bug in the VCL.

BTW, this is on Windows XP.

Workaround:

I have debugged this using Delphi 2009 on Windows XP 64, and the root cause for the missing accelerators seems to be that Windows sends WM_DRAWITEM messages with the ODS_NOACCEL flag set, which it shouldn't if the system is set to show accelerators at all times. So you could say that it is not a VCL bug, but a Windows problem which the VCL does not work around.

However, you can work around it in your own code, you just need to reset the flag before passing the message to the VCL. Override the window proc

protected
  procedure WndProc(var Message: TMessage); override;

like so:

procedure TYourForm.WndProc(var Message: TMessage);
const
  ODS_NOACCEL = $100;
var
  pDIS: PDrawItemStruct;
  ShowAccel: BOOL;
begin
  if (Message.Msg = WM_DRAWITEM) then begin
    pDIS := PDrawItemStruct(Message.LParam);
    if (pDIS^.CtlType = ODT_MENU)
      and SystemParametersInfo(SPI_GETKEYBOARDCUES, 0, @ShowAccel, 0)
    then begin
      if ShowAccel then
        pDIS^.itemState := pDIS^.itemState and not ODS_NOACCEL;
    end;
  end;
  inherited;
end;

This is demonstration code only, you should not call SystemParametersInfo() every time a WM_DRAWITEM message is received, but once at program start, and then every time your program receives a WM_SETTINGCHANGE message.

There is an option for this called "Assignable typed constants" in Compiler Options. It can also be enabled with "{$J+}" in your code.