I am reporting this yet one more time, as it still remains unfixed in 822. It first appeared in 820, and does not appear in 819. As you have released 821 and 822 I've tried them without success (although the README advised me you still have not addressed and fixed the bug, so I'm not really surprised), which is unexpected since this is really a major major bug. I am forced to revert to 819, which works perfectly.
You asked if this defect occurs using "default settings", and the answer is YES... with hardly any tweaks from absolute initial default. Of course I do have to make just a few changes from absolute initial default:
(a) the default presentation for split screen mode is LEFT/RIGHT and I want OVER/UNDER, I use CTRL+H after installing FCXE.
(b) the default "Tree" is not active present, and I want to see both Tree and Details panes in each panel. So I activate the "tree" pane using ALT+T in both the top and bottom panels when in split-screen mode.
(c) the default Tree presentation is "one tree for both panels", which is not what I want. So I go into Tools -> Settings -> Tree and change the "File system tree" default value of "one tree for both panels" to instead be "one tree per panel".
That's all I have done from initial installation defaults, and I CAN 100% DEMONSTRATE THE FAILURE (see screenshot sequence below). Each time I use F10 twice (to toggle from split-screen over/under 2-panel mode back to 1-panel mode, and then to toggle back to split-screen over/under 2-panel mode) the vertical divider which separates the Tree pane from the Details pane in the lower panel shifts to the right by what seems to be a constant "single unit of right-shift". Repeat F10 twice and the divider shifts yet one more time to the right, again by another "single unit of right-shift".
Again, THIS DOES NOT OCCUR WITH 819 nor with any prior version of FCXE x64 Setup that I have been using for the past decade. It only started with 820.
NOTE: I have not changed the "Favorite items tree" setting from its default of "one tree for both panels". I have only changed the "File system tree" to "one tree per panel". I don't know if this is relevant, but I didn't change "Favorite items tree".
The program starting with 820 is unusable for me in this condition because I fairly commonly go back and forth between 1-panel and 2-panel mode using F10 to accomplish this. And each time I return to 2-panel over/under split-screen mode that lower panel divider between Tree and Details panes has shifted right another increment.
Here is the "proof".
(1) Initial window upon first launch immediately following install of FCXE x64 Setup (Win7 x64, for this demo).

(2) After CTRL+H to produce OVER/UNDER split-screen mode, and resizing of the program window to make it larger.

(3) After enabling TREE via ALT+T (note initial default is "one tree for both panels").

(4) After Tools -> Settings -> Tree -> change default of "File system tree" from "one tree for both panels" to "one tree per panel".


(5) Use F10 once, to toggle from split-screen OVER/UNDER 2-panel mode to 1-panel presentation.

(6) Use F10 again, to toggle back from 1-panel presentation to 2-panel OVER/UNDER split-screen.

NOTE in the last screenshot that the vertical divider in the lower panel between the Tree (on the left) and Details (on the right) has "shifted right one incremental unit". And each time I again press F10 twice (toggling to 1-panel mode and then back to 2-panel mode), that lower panel vertical divider will shift to the right yet another "one incremental unit".
Also, attempting to correct the right-shift by dragging that lower-panel vertical divider to the left (to where it originally was) is only remembered temporarily. If F10 is again used twice the vertical divider will return to its last right-shifted location, i.e. this first F10 time it will not right-shift one more incremental unit, but it will simply return to where it last was. But if I now use F10 twice again, sure enough the progressive right-shift will now happen once again. Etc., etc.
THIS IS THE BUG!!! Doesn't matter what is actually in the Details pane or what the view is in that Details pane, or if any specific folder or drive is selected in the Tree pane. That vertical divider will simply continue to right-shift every time F10 is pressed twice and the OVER/UNDER split-screen 2-panel mode is re-presented.
Just in case it might be relevant, I do have my Desktop properties text size set to 125%.