In version 8.x we have the Center=true property which centered the window.
Now with the version 9.1 and Top + Left at 50% the window starts centered, but if we dynamically change the content inside of the window, the window expands to down-right and stopped to be centered.
I had to use a css to force the window to be always centered, but it's hack I don't like.
This is the old version:
<TelerikWindow Modal="true" @bind-Visible="@IsModalTaxValidateVisible" Resizable="true" Centered="true" MaxWidth="700px">
The new version:
<TelerikWindow Modal="true" @bind-Visible="@IsModalTaxValidateVisible" Resizable="true" Class="div_centered" MaxWidth="700px" Top="50%" Left="50%">