[BetterRed]

@Paulie_D, ShellShock - If you're game try this :

1. close calibre, editor etc, rename the configuration folder from %APPDATA%\calibre to say %APPDATA%\calibresave,
   
2. run the editor standalone, change the panel sizes, close editor and open again; this will create a new config folder that will have something like what you see in the attachment
   
3. if the new panel sizes stick, then there is obviously something awry with one of those files in calibresave. My guess is that it will be tweak_book_gui.json.
   
4. so you could replace the tweak_book_gui.json in the calibresave folder with freshly created one, then trash the calibre folder, rename calibresave back to calibre and see how it goes.

BR