SUMMARY
The folder dialog has an area on the left where one can pin/bookmark folders one deals with frequently. This gives one-click access to those folders. This region needs to be resizeable and the max resize width should be much greater.

STEPS TO REPRODUCE
1. Open any KDE program that uses the KDE folder dialog (such as Dolphin's "Moveto /Copy to..." choices) and begin moving or copying something to a folder so that you're prompted about which folder to work with.
2. Notice that the pinned or bookmarked folders on the left are in an area which cannot be resized. Try dragging the area where one expects to find a resizeable border. 

OBSERVED RESULT
Instead one ends up moving the panel.

EXPECTED RESULT
A resizeable border line appears and allows one to resize the bookmark/pinned-folder area to whatever width the user desires. This width is stored (perhaps as a percentage of panel width) and drawn that way by default on subsequent panel openings.

SOFTWARE/OS VERSIONS
Operating System: Debian GNU/Linux 
KDE Plasma Version: 5.17.5
KDE Frameworks Version: 5.70.0
Qt Version: 5.14.2
Kernel Version: 5.7.0-1-amd64
OS Type: 64-bit
Processors: 8 × AMD FX(tm)-8150 Eight-Core Processor
Memory: 31.3 GiB of RAM

ADDITIONAL INFORMATION
The directory panel used to have the folder area as a resizeable area but it could never be resized wide enough to accommodate long folder or volume names. Hence I suggest that the folder area width be as wide as the user wants it to be.