Description:
A box error message occurs when I want to go back from the last cell of line 1 (cell XFD1)to A1048576 cell.
The message says "Libreoffice 7.0 - Fatal error. multi_type_vector::position#1774; block position not found! (logical pos=15999970, block size 1, logical size=1048576)

Steps to Reproduce:
1. Enable big worksheets (Tools/Options/Calc/Settings: 16m lines/16384 col)
2. In "Calc" module, go to XFD1 cell, using, for example, the shortcut CTRL+Right arrow
3. Press CTRL+left arrow;
4. Press CTRL+down arrow.

Actual Results:
A box error pops up:
Title: Libreoffice 7.0 - Fatal error
Message: multi_type_vector::position#1774; block position not found! (logical pos=15999970, block size 1, logical size=1048576)

Expected Results:
Going down to A1048576 cell, without any error


Reproducible: Always


User Profile Reset: No



Additional Info:
Once the box has appeared, it is necessary to press OK button. Then LibreOffice Calc automatically shuts down and you must restart LibreOffice.