Keep the in-cell editor wide enough for the next character at high zoom (#2336)

crates/ui-egui/src/grid.rs

@@ -3087,6 +3087,13 @@ fn keyboard(app: &mut SheetApp, ctx: &egui::Context, resp: &egui::Response, geo:
     }
 }
 
+/// Room the in-cell editor keeps right of its text. It is sized from the text of the last frame, so
+/// the next typed character must still fit: a fixed 14 px was less than one glyph at high zoom,
+/// and the field scrolled and snapped back on every key (#2336). `size` is the font size in px.
+fn editor_slack(size: f32) -> f32 {
+    14.0 + size
+}
+
 fn in_cell_editor(app: &mut SheetApp, ui: &mut egui::Ui, geo: &Geo, sh: &Sheet, wb: &Workbook) {
     let Some(ed) = app.editor.as_ref() else { return };
     if ed.sheet != wb.active_sheet {
@@ -3113,7 +3120,7 @@ fn in_cell_editor(app: &mut SheetApp, ui: &mut egui::Ui, geo: &Geo, sh: &Sheet,
         let text_w = ui.painter().layout_no_wrap(ed.text.clone(), font.clone(), Color32::BLACK).size().x;
         // Excel keeps the cell's height for one line and only grows for extra lines.
         let h = if lines > 1.0 { (size * 1.3 * lines + 4.0).max(rect.height()) } else { rect.height() };
-        ((text_w + 14.0).max(rect.width()).min(geo.cells.right() - rect.left()), h)
+        ((text_w + editor_slack(size)).max(rect.width()).min(geo.cells.right() - rect.left()), h)
     };
     // Grows to the left on a right-to-left sheet.
     let erect = geo.mirror_rect(Rect::from_min_size(rect.min, vec2(w, h)));
@@ -4002,6 +4009,15 @@ pub(crate) fn heading_font_size(zoom: f32) -> f32 {
 #[cfg(test)]
 mod tests {
 
+    #[test]
+    fn editor_slack_fits_one_more_glyph_at_high_zoom() {
+        use super::editor_slack;
+        // A glyph is at most about one em wide; at 400% an em is ~60 px, far over the old fixed 14 px.
+        for size in [14.7_f32, 25.0, 60.0] {
+            assert!(editor_slack(size) - 4.0 >= size, "slack {} too small for a {size} px glyph", editor_slack(size));
+        }
+    }
+
     #[test]
     fn explicit_left_and_right_keep_their_side_on_a_right_to_left_sheet() {
         use super::{HAlign, painted_halign};