Jump to line broken in the debugger

When selecting a breakpoint or a stack frame when paused in the debugger, the source view does not jump to the proper line. In some cases, it jumps incorrectly to the bottom of the file, and in other cases it doesn't jump at all.
@elliette

Start by reproducing the issue in the debugger described in the report: pause execution, then select a breakpoint or stack frame and observe the source view. Trace the navigation behavior and verify that each selection reaches the proper source line, never the bottom of the file, and never fails to move.

索引モデルが issue の本文から書いたものです。