The normal overscan_left/overscan_right/overscan_top/overscan_bottom only apply to the framebuffer (e.g. console and X gui). They have never applied to other overlays, like video or opengl (which are used by Kodi).
overscan_scale=1 is a hack that I don’t recommend using. It attempts to scale all the other overlays (including video and opengl). But it is a bad thing to do as the host application doesn’t know about it.
Lets imagine you have a native 1280x720 display, but it is badly set up such that only the central 1200x640 pixels are visible. You have two choices here. You can use the calibration option in Kodi’s gui, which will result in the GUI being rendered at a 1200x640 and output unscaled. Or you can use overscan_scale=1 which results in the gui being rendered at 1280x720 and being rescaled by the Pi to 1200x640. This scaling will degrade the image and make it look more blurry.
Because overscan is configured in TV you will likely get an additional resize to it’s native resolution. You need to disable overscan in the TV menus to avoid that, but the presence of “overscan_scale=1” is adding an extra rescale to the displayed image.
There was a firmware fix 12 days ago where the overscan settings were incorrectly applied. The behaviour before then was a bug, so it may be that you have to update the overscan_* settings you are using to compensate for that. However I believe the new settings are correct, and the old settings were incorrect.
In general my recommendation would be in order of preference:
- Ensure a 1:1 scaling in TV and disable all overscan/calibration settings. This gives you the best image quality. Video that matches native resolution of TV will be unscaled. Other sizes will be resized once by Pi.
- Use the calibration settings inside kodi. The GUI will be resized by TV once, but not by Pi. Video will likely be resized twice (by Pi and TV).
- Use overscan_scale. The GUI will be resized by TV and again by Pi. Video will likely be resized twice (by Pi and TV).