fix(FillSpace): prevent aboveRef, belowRef, and onMeasurement from leaking to DOM

src/dev/mock-pages/FillSpaceMockPage.jsx

@@ -16,9 +16,11 @@ class FillSpaceMockPage extends Component {
     this.ex1 = { header: createRef(), footer: createRef() };
     this.ex2 = { header: createRef(), footer: createRef() };
     this.ex3 = { header: createRef(), footer: createRef() };
+    this.state = { fitCount: null };
   }
 
   render() {
+    const { fitCount } = this.state;
     return (
       <Box sx={{ display: 'flex', flexDirection: 'column', gap: 3 }}>
         <Typography variant="body2" color="text.secondary">
@@ -54,6 +56,7 @@ class FillSpaceMockPage extends Component {
                   <Typography variant="body2">Einheit</Typography>
                 </Box>
               }
+              onMeasurement={(count) => this.setState({ fitCount: count })}
             >
               <Box
                 sx={{
@@ -69,6 +72,29 @@ class FillSpaceMockPage extends Component {
                   Diese Zeilen stehen oben, darunter wiederholt sich die
                   Einheit bis zum Seitenende.
                 </Typography>
+                <Box
+                  sx={{
+                    mt: 1,
+                    px: 1.5,
+                    py: 0.75,
+                    borderRadius: 1,
+                    bgcolor: 'background.paper',
+                    border: '1px dashed',
+                    borderColor: 'divider',
+                  }}
+                >
+                  <Typography variant="caption" component="span">
+                    📏 onMeasurement (würden ohne Inhalt passen):&nbsp;
+                  </Typography>
+                  <Typography
+                    variant="caption"
+                    component="span"
+                    fontWeight={700}
+                    color="primary"
+                  >
+                    {fitCount != null ? `${fitCount} Einheiten` : '…'}
+                  </Typography>
+                </Box>
               </Box>
             </FillSpace>