Hi,
Base on your description, you used the master page and the menu control is on the left side of the master page, you want to fix the menu control when you scroll the creen, right?
I would suggest you can try to put the contentplaceholder in a div, and give the div a fixed height and width and set the div's overflow:scroll, therefore, you can only scroll the area of the content page, the other part of the master page is fixed, therefore the left menu is also fixed when you scroll the screen.
For example:
<div style="overflow:scroll;height: 283px; width: 468px;" >
<asp:ContentPlaceHolder ID="Main" runat="server">
</asp:ContentPlaceHolder>
</div>
Hope it helps.
Please remember to mark the replies as answers if they help and unmark them if they provide no help.
Yours sincerely,
Amanda Wang
Microsoft Online Community Support