Because you can.
This plugin overrides renderer.draw_rect and renderer.set_clip_rect to check if any draw calls occlude the ImgView. If it is occluded, the image will be rerendered. As you expect, this is slow and it will slow down your editor even when not in use.
The image is rendered pixel-by-pixel with renderer.draw_rect. The renderer doesn't attempt to be smart by merging adjacent pixels, so it will be very slow.
This is actually a QOI viewer. However, it does support other common image formats like PNG and JPGs with a converter.
This repo also contains a QOI decoder modified from SloppyQOI. This modified decoder can decode image on-the-fly with an iterator based API.
Grab the plugin from Releases and extract it in USERDIR/plugins/img. USERDIR is usually ~/.config/lite-xl.
$ gcc -o img img.c -lm- resize images
- moving image around
- animation
- actually display image (without hacks)