App does not remember separator position

[rex4539]

insider-20230112-1 Reproducibility: always

Steps:

1. Drag the column separator and make the middle pane bigger.
2. Quit the app.
3. Relaunch the app.

What happened: Separator is reset to default.

Expected result: Separator is positioned where it was left before quitting the app.

[livid]

Thanks! We will look into this.