Here are a couple screenshots I used with the XGA preset in DOSBox Pure without any settings changes, except for Aspect Ratio Correction to ON. This is what you should see when using the XGA preset with typical DOS programs. If you use VGA, the scanlines will be much weaker. This is normal and expected, as XGA monitors have narrower beams than VGA monitors.
@lfan, the issue you see is because you are using integer scale and ovescale. Turn those off.
@alexb3d, I suspect the issue you have with Basic is because that preset is for NTSC use, not computer use, and so the image is far too zoomed in then it should be. Try the preset I made below which turns the zoom off. It also aligns the other settings perfectly with the XGA preset.
shaders = "1"
shader0 = "src/scanline-basic.slang"
filter_linear0 = "false"
scale_type0 = "viewport"
scale0 = "1.0"
TRANSFER_FUNCTION = "1.0"
COLOR_MODE = "3.0"
LUMINANCE_WEIGHT_R = "0.2126"
LUMINANCE_WEIGHT_G = "0.7152"
LUMINANCE_WEIGHT_B = "0.0722"
CHROMA_A_X = "0.64"
CHROMA_A_Y = "0.33"
CHROMA_B_X = "0.30"
CHROMA_B_Y = "0.60"
CHROMA_C_X = "0.15"
CHROMA_C_Y = "0.06"
CHROMA_A_WEIGHT = "0.2126"
CHROMA_B_WEIGHT = "0.7152"
CHROMA_C_WEIGHT = "0.0722"
SCALE_W = "1.0"
SCAN_TYPE = "1.0"
MAX_SCAN_RATE = "768.0"
LINE_DOUBLER = "1.0"
INTER_OFF = "0.0"
FOCUS = "0.12"
ZOOM = "1.0"
COLOR_SPACE = "1.0"
To change this preset to match the VGA preset, simply change the MAX_SCAN_RATE to 480, and optionally change SCAN_TYPE to 2.0. As you increase MAX_SCAN_RATE, you will get a stronger scanline effect.