The TB_INDETERMINATE message sets or clears the indeterminate state of the specified button in a toolbar.
TB_INDETERMINATE
wParam = (WPARAM) idButton;
lParam = (LPARAM) MAKELONG(fIndeterminate, 0);
Parameters
idButton
Command identifier of the button whose indeterminate state is to be set or cleared.
fIndeterminate
Indeterminate flag. If this parameter is TRUE, the indeterminate state is set. If it is FALSE, the state is cleared.
Return Values
Returns TRUE if successful or FALSE otherwise.