To be more precise, F11 is used to force the render window to the background (or bring it to the foreground on the second press).
Border rendering is working fine here. (Windows)
In camera view (NumPad 0), first toggle the ‘Border’ button, then press Shift_b and draw a rectangle around the area you want rendered. It should leave a red-dashed outline indicating the selected area to render. Then press F12 or the ‘Render’ button.
Otherwise, I don’t know. Could be a bug. What OS version of Blender are you using if that’s the case?
Precise ?? Yes, of course, ONLY if you have pressed DispWin button (Scene - F10). BUT if you press DispView you don’t have this render window
In camera view (NumPad 0), first toggle the ‘Border’ button, then press Shift_b and draw a rectangle around the area you want rendered. It should leave a red-dashed outline indicating the selected area to render. Then press F12 or the ‘Render’ button.
Border button worked fine (in previous versions) with no need to use Shift-b
Otherwise, I don’t know. Could be a bug. What OS version of Blender are you using if that’s the case?