StoryBook window allows excessive horizontal resize and left section shows empty content

Steps to reproduce:

1. Create a new Unity Project
2. open Window → App UI → StoryBook
3. Resize the App UI – StoryBook window horizontally
4. Observe the layout behaviour of the left and right sections

Actual results: The left section shows empty content with no minimum width constraint

Expected results: Layout should remain consistent and usable when resizing horizontally

Reproducible with versions: 1.3.1 (6000.2.2f1), 2.1.1 (6000.3.0a5)

Can’t test with versions: 6000.1.16f1 (no App UI)

Tested on (OS): macOS Silicon 15.6 Beta (24G5054d)