System Settings -> Display and Monitor Recently helped a friend with vision difficulties scale up their desktop elements/fonts to avoid eye strain(they had a condition that had optic nerve damage where some vision was restored but is still quite limited). While not HiDPI exactly, I think the scaling is still useful in this case for the 1080p display? Cursor size and Font DPI were also increased. In this settings window, I used the Scale Display button to try find a comfortable size for the user. If the scale value was too high(I think 1.7x-2x) some windows/dialogs were too big vertically, so while it was a more ideal scale for them I had to reduce it as a particular dialog(native KDE app I think, sorry can't recall what one) had it's apply/cancel buttons cut off, and the titlebar likewise was cropped off desktop, I couldn't seem to reposition/resize the window(I think there is a key shortcut for this but I did not know it offhand nor expect a casual user to know about such and remember it). Some applications icons had a pixelated like look instead of a crisp aesthetic. I understand scaling especially non-integer values can be responsible for this on bitmap graphics, though I think it still happened with 2x on Octopi(Qt package manager app). This also seems to affect some text, like when shutting down or rebooting, the overlay with countdown and ability to change your choice, the text here appeared pixelated. Yakuake was affected by it's bar with close/menu/pin buttons, as well the red X above that. Taskbar scaled great, as did window decorations(titlebar buttons here were crisp). I believe I noticed that this scale value was also adjusting Font DPI as when we went to tweak it was at a value I don't recall setting, that was unexpected since we started with Font DPI first, I guess that wasn't required. --- Kudos to devs as the experience was less painful than expected. I was mostly fine with google queries and exploring system settings, KDE content on accessibility for such things is a bit spread out and dated from what I gathered with findings described here: https://www.reddit.com/r/kde/comments/6rj708/accessibility_vision_and_scaling/dl5geuu/
Please do not use bugzilla tickets to describe your experiences; this only confuses developers who want to understand the actual bugs. Reading the description, you are describing multiple issues, which cannot be solved using a change in a single component. To assign a ticket to the developers responsible, and managing its status, it is required that a single issue is described in a ticket. Please use one ticket for one issue. Describe it concisely (steps to reproduce, what you expect, what you got instead). If more information is needed to understand the issue, a developer might ask specific questions.