Resizing panes has some issues

[phwelo]

mailspring version: 1.1.0-7563

bar shows along the border between navigation and list, appears to be 4-8px and white.
no bar shows on the border between list and reading pane, which is probably expected.

trying to grab and drag either of these bars behaves incorrectly, causing the other border to resize and sometimes moving in the opposite direction of the cursor