SUMMARY In Kubuntu 18.10 64-bit, when attempting to shrink the width of the places panel to be smaller than the width of the enclosed items, the width will automatically regress (to full size / auto-fit) after I release the mouse button and move the pointer STEPS TO REPRODUCE 1. Open dolphin 2. Re-size the places panel width to be small 3. Release the mouse button 4. Move the pointer a little bit OBSERVED RESULT The places panel resumes it's full width EXPECTED RESULT The places panel remains at the width I just resized it SOFTWARE VERSIONS (available in About System) KDE Plasma Version: 5.13.5 KDE Frameworks Version: 5.50.0 Qt Version: 5.11.1 ADDITIONAL INFORMATION There is a workaround (mostly works). If I do not move the pointer after resize, and immediately close dolphin (using keyboard shortcut), when I restart dolphin the new width is present and will stay. So I think the problem is related to moving the pointer immediately after resize. Places panel width resize worked regular in Kubuntu 18.04 and 18.04.1, so that is why I marked as a regression
Thanks for the detailed steps to reproduce. Kubuntu 18.10 ships with Dolphin 18.04.3, but I cannot reproduce with Dolphin 18.08.x or git master. Looks like it's already been fixed.