How to set default position for bars
As shown in above figures, I can manually drag these bars to rearrange them.
The problem is that this rearrangement is lost whenever I restart TeXmacs, How to make the new rearrangement as default.
It may or may not be the file you are looking for, but please try and have a look at .TeXmacs/system/preferences.scm
If you do not get any answer here, keep in mind that Joris van der Hoeven reads the mailing list more than the forum (which on the other hand has sufficiently different characteristics that having both a forum and a mailing list is good), so a try on the mailing list might also be helpful.
I don’t think this kind of preference has been implemented.