@if (!IsConnecting)
{
   <a href="javascript:void(0)" @onclick="Connect">Connect</a>
}
else
{
   <button disabled>Connecting...</button>
}
@code {
bool IsConnecting { get; set; }

private async Task Connect()
{
   IsConnecting = true;
   StateHasChanged();
   //Long operation -> it takes a few seconds
   IsConnecting = false;
   StateHasChanged();
}
}
// The disabled button is not shown
// If I put a return after the first StateHasChanged() -> works
// If I don't reset IsConnecting to false -> works
// It seems StateHasChanged() is not called *during* the execution of the method. Only when the function is completed. Why? :(