flex-basis is the size before flexing
A two-pane editor looked right on the designer's monitor and wrong on everyone else's — the sidebar kept a constant *percentage* when it was supposed to keep a constant *offset*.
Two panes, filling the split with no seam. At the current 600px they have to be 400 and 200. That much is easy. Here is the part that decides whether you have it right: the left pane must stay exactly 200px wider than the right at every width. Widen the split by 200px and each pane grows by 100. The difference never changes. The starter already does half the job. Read what it renders before you change it.
Hint
The starter gives each pane a starting size and stops. What happens to the space that is left over, and who is entitled to it?
The fixture — markup and base stylesheet
You cannot change either of these. They are what your selectors have to reach, and they are the same ones the judge renders.
<div class="split"><div class="pane a"></div><div class="pane b"></div></div>
*, *::before, *::after { box-sizing: border-box; } html, body { margin: 0; padding: 0; } body { background: #0b0b0f; } .split { width: 600px; height: 140px; background: #16161d; } .pane { height: 140px; } .a { background: #ffd166; } .b { background: #7bdff2; }