Wrong dock position in multiple display setup

I use laptop PC with HDMI connected external display. Resolution of internal screen is 1920x1080 (I don't use it, it is off) and resolution of my main external screen is 1920x1200. On my main screen dock floats 120 pixels above screen bottom edge as if resolution of the screen is 1920x1080:

No source files, tests, or entry points are named. Reproduce the setup with a 1920x1080 internal display and a 1920x1200 external display, then trace how the dock determines the active display’s bounds and bottom edge. Done means the dock reaches the external display’s actual bottom edge in this configuration.

Written by the indexing model from the issue text.