SUMMARY I have two screens, both 1080p, but one (left, main) non rotated, and the other on the right, rotated 90° to the left. I have my main panel on the left side of the left screen, and I wanted to create a new panel for the bottom of the screen in the right. When I created a new empty panel from the first panel's config menu, it created it in the left screen. Then I moved it to the bottom of the right screen and added a task manager (not the icons-only one). I configured it to have only one row, no grouping, and not filtering by desktop (I have 6). The task list run off the right edge and I couldn't reach most of my tasks (I had at least 14, maybe closer to 20). It took me a while to find a point in the panel where I could right click (where did the cashew go?) and replace the task manager with a icons-only one. Now I can see all my tasks, but they're sorted by PID or something and not by desktop, but that's another bug. Please point me to the right configuration files to see if the panel size is correct and whether editing it by hand can fix this.
This seems to be fixed in Plasma 5.26.1. I can move a panel with a task manager around, either by dragging or using the Panels and Desktops Management dialog, without the task manager flowing outside the panel. The task manager does still have some weird behavior after switching orientations from dragging like using columns in a horizontal panel when 1 row is specified, but that's unrelated.