MultipleChoice: I've tested your original example as well as the one you posted in the bug report test_example.capx. This "shaking" you're referring to, the problem with the "shaking", is it possible that this is a v-sync / graphical glitch?
Here are some test cases you should try out:
1. Turn a small layout, disable camera following code
2. Pixel rounding off / on
3. Sampling - Linear/Point
In which combination of these options does the problem go away?
If we can narrow the problem down to the scrolling code, maybe try only moving the camera when the difference between (Scrollx, scrolly) and (PlayerX, PlayerY) is > 1 pixel. Also try using a lerp for the camera to see if it smooths out the scrolling any.
Just set a local variable CameraSpeed to 1/2.
Scroll to position:
lerp(ScrollX, PlayerBox.X, dt * CameraSpeed)
lerp(ScrollY, PlayerBox.Y, dt * CameraSpeed)