[JimmXinu]

Yes, it does. I also deleted gprefs['book_list_pin_splitter_state'] and then tried again with `calibre --ignore-plugins` and it still re-centered.

I added some debug output and save_state is being called during startup before restore_state. (calibre-debug doesn't support --ignore-plugins, BTW)

Debug output:

Spoiler:

calibre 3.17* [64bit]  embedded-python: True is64bit: True
Windows-10-10.0.16299-SP0 Windows ('64bit', 'WindowsPE')
('Windows', '10', '10.0.16299')
Python 2.7.12+
Windows: ('10', '10.0.16299', 'SP0', u'Multiprocessor Free')
Interface language: None
Successfully initialized third party plugins: DeDRM (6, 5, 2) && SmartEject (2, 0, 7) && Count Pages (1, 8, 0) && FictionDB (1, 0, 10) && fanficauthors.net CSS Fix (0, 0, 2) && Generate Cover (1, 5, 21) && Unplugged (1, 0, 0) && FanFicFare (2, 22, 2) && Webscription (1, 1, 0) && ColumnSum (2, 0, 0) && EpubSplit (2, 3, 0) && View Manager (1, 4, 3) && Reading List (1, 6, 6) && EpubMerge (2, 1, 1) && Quality Check (1, 9, 11) && Find Duplicates (1, 6, 3) && Modify ePub (1, 3, 13) && Barnes & Noble (1, 2, 14) && Favourites Menu (1, 0, 4) && Kindle 2, 3, 4, Touch, PaperWhite, Voyage Device Interface MBP Update Mod (1, 0, 0) && Manage Series (1, 2, 8)
Turning on automatic hidpi scaling
devicePixelRatio: 1.0
logicalDpi: 96.0 x 96.0
physicalDpi: 141.76744186 x 141.402061856
Using calibre Qt style: True
Starting up...
save_state:
PyQt5.QtCore.QByteArray('\x00\x00\x00\xff\x00\x00\x00\x01\x00\x00\x00\x02\xff\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\x01\x00\x00\x00\x01\x00')
restore_state():
bytearray(b'\x00\x00\x00\xff\x00\x00\x00\x01\x00\x00\x00\x02\xff\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\x01\x00\x00\x00\x01\x00')
restoreState:True
save_state:
PyQt5.QtCore.QByteArray('\x00\x00\x00\xff\x00\x00\x00\x01\x00\x00\x00\x02\x00\x00\x00F\x00\x00\x00F\x00\xff\xff\xff\xff\x01\x00\x00\x00\x01\x00')
Started up in 4.08 seconds with 17 books

I threw a quick lock boolean in to prevent saving before restoring and now the split bar is staying where I put it across restarts.

Kludged & debug PinContainer:

Spoiler:

import pprint
pp = pprint.PrettyPrinter(indent=4)

class PinContainer(QSplitter):

    def __init__(self, books_view, parent=None):
        QSplitter.__init__(self, parent)
        self.setChildrenCollapsible(False)
        self.books_view = books_view
        self.addWidget(books_view)
        self.addWidget(books_view.pin_view)
        books_view.pin_view.splitter = self
        self.has_loaded = False

    def save_state(self):
        print("save_state:")
        if self.has_loaded:
            state = bytearray(self.saveState())
            pp.pprint(self.saveState())
            gprefs['book_list_pin_splitter_state'] = state
        else:
            print("DONT save yet")

    def restore_state(self):
        val = gprefs.get('book_list_pin_splitter_state', None)
        print("restore_state():")
        self.has_loaded = True
        pp.pprint(val)
        if val is not None:
            print("restoreState:%s"%self.restoreState(val))

I can clean that code up and submit it if you find it an acceptable workaround, but it doesn't explain why it happens in the first place.