We are using "Scale outer" for "Fullscreen in browser" mode, which makes enormous differences in where the "edges" of the screen are relative to the internal coordinate system in Construct.
I have found this to work fairly reliably across different modes:
CanvasToLayerY("backdrop",0,0) // top of screen
CanvasToLayerY("backdrop", WindowWidth, WindowHeight) // bottom of screen
CanvasToLayerX("backdrop",WindowWidth, WindowHeight) // right side of screen
CanvasToLayerX("backdrop",0,0) // left side of screen
I use these to make my user interfaces in Construct fully dynamic (i.e. if the user resizes the window, Construct will automatically resize everything inside).
Note: "backdrop" is my primary display layer's name.