[Keybindings GUI] wrong hover on when context 

Issue Type: <b>Bug</b>

1. Open Keybindings GUI
1. (Sometimes optional) Scroll down/up
1. Hover over empty **when** columns `—` 
1. It shows **when** contexts from some other rows

VS Code version: Code - Insiders 1.31.0-insider (8f56013e8f66650c1b0b65cc5c502691e307db21, 2019-02-01T18:34:04.940Z)
OS version: Windows_NT x64 10.0.17763