Interesting bug report.
I've just done some tests and I'm fairly sure the problem is connected in some non obvious way with the window resolution used in your project settings.
I could reproduce your black screen whenever I used values where at least one dimension was larger than the window resolution set in the project.
For example, my default project setting is 640x480. Your code "works" (i.e. no black screen) if I use values such as
copy bitmap OffscreenBitmapNumber,0,0,482,790,BitmapNumber,0,0,640,480
but I get the black screen as soon as I change that to
copy bitmap OffscreenBitmapNumber,0,0,482,790,BitmapNumber,0,0,641,480
What seems to be happening is that your window is being interpreted as a bitmap with dimensions corresponding to your project settings - but the actual size on screen is measured in pixels relating to the desktop resolution. So, using my default settings with a window resolution of 640x480 the copy bitmap command seems to treat your 1280x960 sized window as a bitmap with size 640x480 and scales accordingly. Hence the following rescales the original 482x790 image to fill the window:
copy bitmap OffscreenBitmapNumber,0,0,482,790,BitmapNumber,0,0,640,480
If I use the following instead I get a small black border on the right hand side and along the bottom:
copy bitmap OffscreenBitmapNumber,0,0,482,790,BitmapNumber,0,0,635,475
Hope this makes sense.
This is probably not a bug - more likely just the way the commands work.
If I change the project window resolution to something else, say 1600x900, then the problems occur when the target bitmap width or height resolutions exceed 1600 or 900 respectively.