Enable the Information pane in Dolphin. Select a file with a very long name, such that the file's name appears in multiple lines in the Information Pane. Resize the Information page wide enough so that the filename unwraps to a single line. Now you can't shrink the Information Pane back to a reasonable size. At least, not until selecting another, shorter-named file.
I am not able to reproduce this bug on KDE Neon 5.11.5 with Dolphin 17.12.1. Could you please check again if this bug still persists? If yes, could you tell us which distribution you are using Dolphin on? Thanks for helping to triage this bug :)
I can not reproduce on Arch Linux, Dolphin 17.12.1.
Closing, can no longer be reproduced.