Run this to cut the glyph off at y=0 and either x-height or cap-height, depending on the glyph.

slice_top_and_bottom.py

# menuTitle: Slice Top and Bottom of Glyph - EZUI

import ezui
from mojo.tools import IntersectGlyphWithLine
import math

oob = 1000 # out-of-bounds distance

def intersectContourAtHeight(g, height):
    
    slicer = g.getRepresentation(
        "doodle.Beam", 
        beam=((-oob, height), (g.width + oob, height)), 
        canHaveComponent=True
        )

    slicer.sliceGlyph(roundValue=True, addIntersectionsOnly=False)
        
            
    # if the height is low, delete all the low points
    if height <= 0:
        for c in g.contours:
            if c.bounds[3] == height:
                g.removeContour(c)
                
            # if slice didn't make distinct contours, get rid of points
            for pt in c.points:
                if pt.y < height:
                    c.removePoint(pt)
        
                
    # if the height is high, delete all the high points
    else:
        for c in g.contours:
            if c.bounds[1] == height:
                g.removeContour(c)
                
            for pt in c.points:
                if pt.y > height:
                    c.removePoint(pt)                
                

class SliceGlyph(ezui.WindowController):

    def build(self):
        content = """
        ( ) Ascender     @topCut
        (X) Cap-Height
        ( ) X-Height
        --------
        (X) Baseline     @bottomCut
        ( ) Descender
        --------
        (Slice Glyph)    @commit
        """

        self.w = ezui.EZWindow(
            content=content,
            title="Slice Off",
            size=(140, 'auto'),
            controller=self
        )

    def started(self):
        self.w.open()

                    
    def commitCallback(self, sender):
        f, g = CurrentFont(), CurrentGlyph()
        
        top = {
            0: f.info.ascender, 
            1: f.info.capHeight, 
            2: f.info.xHeight,
            }
            
        bottom = {
            0: 0, 
            1: f.info.descender
            }
            
        heights = [top[self.w.getItemValues()['topCut']], bottom[self.w.getItemValues()['bottomCut']]]

        with g.undo(f"Slice Glyph {g.name}"):
            for height in heights:        
                intersectContourAtHeight(g, height)

            g.changed()

SliceGlyph()