I don't know if this helps you, but perhaps you can try it:

I have a small web frontend for VirtualBox that creates RDP files. Those RDP files, when started, open a Terminal Service Client window, not in fullscreen mode, but as a simple window. I don't know if thes show the same behavior with a Microsoft Terminal Server, but I think it is worth trying it.

This is one of the files, set up for 1024x768, connecting to 192.168.2.10 Port 3011 (default=3389):

screen mode id:i:1
desktopwidth:i:1024
desktopheight:i:768
session bpp:i:32
winposstr:s:0,1,1291,12,2558,1001
compression:i:1
keyboardhook:i:1
displayconnectionbar:i:1
disable wallpaper:i:0
disable full window drag:i:0
allow desktop composition:i:1
allow font smoothing:i:1
disable menu anims:i:0
disable themes:i:0
disable cursor setting:i:0
bitmapcachepersistenable:i:1
full address:s:192.168.2.10:3011
audiomode:i:0
redirectprinters:i:0
redirectcomports:i:0
redirectsmartcards:i:0
redirectclipboard:i:1
redirectposdevices:i:0
autoreconnection enabled:i:1
authentication level:i:0
prompt for credentials:i:0
negotiate security layer:i:1
remoteapplicationmode:i:0
alternate shell:s:
shell working directory:s:
gatewayhostname:s:
gatewayusagemethod:i:4
gatewaycredentialssource:i:4
gatewayprofileusagemethod:i:0
promptcredentialonce:i:1
drivestoredirect:s:
[download]

Edit the full address line, save as text, rename to *.rdp, double click to open.

Alexander