Grid: row and column heading labels grow with the zoom

At high zoom the heading labels stopped at 16 points (the outline levels
at 14) while the cells kept growing, so at 300-400% the letters and
numbers looked tiny in their tall headings. Excel scales them with the
zoom. The labels now scale without a cap and keep their readable floor
at low zoom (heading_font_size).

Signed-off-by: MH <q6rrzz@gmail.com>

crates/ui-egui/src/grid.rs

@@ -1701,7 +1701,7 @@ pub(crate) fn paint_headers(
     t: &Tokens,
     quads: &[((u32, u32), (u32, u32), Rect)],
 ) {
-    let font = theme::ui_font((11.5 * (geo.z / DISPLAY_SCALE).max(0.75)).min(16.0));
+    let font = theme::ui_font(heading_font_size(geo.z));
     // The headers' own area: inside the outline bands.
     let r = Rect::from_min_max(pos2(geo.head_left(), geo.head_top()), geo.rect.max);
     let top = Rect::from_min_max(pos2(geo.cells.left(), r.top()), pos2(r.right(), geo.cells.top()));
@@ -3336,8 +3336,23 @@ mod selection_fill_tests {
     }
 }
 
+/// Row and column heading labels grow with the zoom as the cells do, as in Excel (at 400% the
+/// labels are four times their 100% size), and never get smaller than a readable 8.5 points.
+pub(crate) fn heading_font_size(zoom: f32) -> f32 {
+    (11.5 * (zoom / DISPLAY_SCALE)).max(8.5)
+}
+
 #[cfg(test)]
 mod tests {
+
+    #[test]
+    fn heading_labels_scale_with_the_zoom_without_a_cap() {
+        use super::{DISPLAY_SCALE, heading_font_size};
+        let at = |percent: f32| heading_font_size(percent / 100.0 * DISPLAY_SCALE);
+        assert!((at(400.0) / at(100.0) - 4.0).abs() < 1e-4, "four times as large at 400%");
+        assert!(at(200.0) > at(100.0) && at(400.0) > at(200.0));
+        assert_eq!(at(10.0), 8.5, "never below a readable size");
+    }
     use super::*;
 
     #[test]

crates/ui-egui/src/outline.rs

@@ -261,7 +261,8 @@ pub fn paint(p: &Painter, geo: &Geo, sh: &Sheet, t: &Tokens) {
             pc.line_segment(tick, ink);
         }
     }
-    let font = theme::ui_font((10.0 * (geo.z / DISPLAY_SCALE).max(0.75)).min(14.0));
+    // The outline levels grow with the zoom like the headings beside them.
+    let font = theme::ui_font((10.0 * (geo.z / DISPLAY_SCALE)).max(7.5));
     for b in &l.buttons {
         let clip = match b.action {
             Action::Toggle { rows: true, .. } => row_clip,