I had the same problem after tiling windows side-by-side - the file list and preview got smooshed to the outer edges, and resizing them didn't stick... it reoccurred every time I opened the editor.
Removing
Code:
"image-editor-state": {
"__class__": "bytearray",
"__value__": "AAAA/wAAAAD9A etc. AAAAAA=="
},
from tweak_book_gui.json unstuck the dividers without whacking all of the other settings.