SUMMARY One can't manually maximize Dolphin: if one starts in lower left, and then drags to top right, when one gets to top, it resizes back down. STEPS TO REPRODUCE 1. Open Dolphin in only part of screen (such as lower left). 2. Drag top right corner to top right. OBSERVED RESULT Can't manually maximize. EXPECTED RESULT Allow manually maximizing rather than resizing downwards. SOFTWARE/OS VERSIONS • Linux/KDE Plasma: Slackware GNU/Linux 15+current, FreeBSD UNIX 14.2 (stable), Devuan GNU/Linux 5 (stable) (all 64-bit), likely most/all current UNIX/GNU/Linux. • KDE Plasma Version: 5.27.12 • KDE Frameworks Version: 5.116.0 • Qt Version: 5.15.16 ADDITIONAL INFORMATION This happened years (maybe since KDE Plasma 4 (KDEP4)) and I had same problem on KDEP6 a few days ago but switched back to KDEP5. I only had this happen on workstation desktop PC with three screens: top 1600x1200, lower 4k, right side '1080p', and put Dolphin on top screen. If you try to maximize other ways (than starting at lower left) you may have even worse results (such as if started at top left and moved down, it may jump to screen below), but that may be common to more than one program; only Dolphin resizes.
Dragging to the top right screen corner won't maximize the window; it will tile it to the top-right screen corner, so what you're seeing seems intentional. To maximize by dragging, you have to drag the window up to the top of the screen but *not* right in a corner.