absorb: align short diff lines with container width

Summary:
When there are 2 diff chunks, a short line, and a long line (wider).
The short one's background color might not fill the width of the column.
This change fixes it.

Reviewed By: evangrayk

Differential Revision: D67980845

fbshipit-source-id: 1613c8605424eb01d738aa818d09457320893085

addons/isl/src/stackEdit/ui/AbsorbStackEditPanel.tsx

@@ -61,6 +61,8 @@ const styles = stylex.create({
     whiteSpace: 'pre-wrap',
     fontFamily: 'var(--monospace-fontFamily)',
     fontSize: 'var(--editor-font-size)',
+    // Fill the width when there are long lines in another diff chunk.
+    flexGrow: 1,
   },
   absorbEditPathTitle: {
     padding: 'var(--halfpad) var(--pad)',