Everytime I open the Jabref, the left column is too wide and the entry details on the bottom shows, leaving the main window very small.
When I drag and resize it, and close the bottom entry's details part, it looks good until I quit and reopen the program again. It seems that the UI failed to remember my adjustment. I have search the settings and found no place to set.

Caption: Too wide on the left, bottom part is annoying, the main part is so small. It occurs every time I start the program.
I can confirm this behavior is occurring. I will further investigate it and report back.
Thanks to @Ka0o0 this should be fixed in the latest development version. Could you please check the build from http://builds.jabref.org/master/. Thanks! Please remember to make a backup of your library before trying-out this version.
@tobiasdiez @Ka0o0 Thanks for the effort, I can confirm that the left panel now can remember its size after closing, but the bottom show details is still occurring even I close it before exit.
@digdig thanks for testing it out. I misunderstood it then. I checked if the bottom panel remembers the size.
I will have a look at it
My investigations have shown that clearAndSelectFirst in BasePanel. It is therefore the intended behavior. @digdig if you feel like this is not how it should be like please open a new feature request issue here on GitHub where we can discuss it separately.
@Ka0o0 Thanks, this behavior is acceptable for me.
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I can confirm this behavior is occurring. I will further investigate it and report back.