Which version of CE are you using?
we have a new feature called viewportMode, to easily setup CE project for multi resolution .
take a look at this example which makes use of it :
We have implemented the strategies described here :
so that you don't have to worry about the viewport rectangle in theory.
with 3.1.8 (or the current source code) that will contain the viewportMode, the default setup will be the old one : the viewport will fill the screen and the starling stage will be of the same dimensions as the viewport.
But if you want to use strategy 3 for example in your case, you can remove all custom code you've added yourself that changes viewport or listens to the resize event (citrus engine already listens to it so it will create conflict, you better override handleStageResize)
and set the following in your Main :
_baseWidth = 1280;
_baseHeight = 768;
_viewportMode = ViewportMode.FULLSCREEN;
your base project is 1280x768.
In a 1920x1080 screen, using this mode and with only this code, CE will take care of fitting the viewport in the screen (no borders) and set the starling stage size to keep the original aspect ratio of the project while still upscaling it correctly.
Basically the goal here was to stop all confusion and let people automatically set up their project for multi resolution with 3 lines.
Note that this mode actually applies the strategy 3 described on the starling wiki page.
ViewportMode.LETTERBOX is strategy 2 , so with borders.
ViewportMode.MANUAL is where you decide on your viewport, and take care of resizing yourself (override handleStageResize) but I highly recomment you try FULLSCREEN because its seems to be what you want.
ViewportMode.LEGACY is the default mode, which basically puts everything at 1:1 with the flash stage.
edit: additional info,
if the camera is following a target too high, you would see things out of bounds. to prevent that, set the camera's bounds property to a rectangle that fits your level so that it will be forced to stay within those bounds and you won't see anything outside of your level.
you might want to try camera.bounds = new Rectangle(0,0,int.MAX_VALUE,768);
with this rectangle camera shouldn't go either up or down. if you want room vertically of course, increase the height of the rectangle but keep it at a y of 0.