when you make the screen go fullscreen, theres 2 stretch modes:

Viewport: the game viewport scales up to fit your monitor, which is the no hdpi mode, means the pixels will stay the same and when the sprite bends or rotates, the pixel will rotate and bend with it

Canvas items: the game's actual resources (sprites, textures) scale up, which makes the sprite's pixels larger, which means when the sprite bends or rotates, the "pixel" has more pixels into it, which makes the sprite bend and rotate smoothly

Canvas items mode requires more performance than viewport mode!