From 14f2ea9f57f98d7c7f4f77b2cc9e97d0f21c618f Mon Sep 17 00:00:00 2001
From: Marco Arnulfo <marcoarnulfo1@gmail.com>
Date: Fri, 16 Oct 2026 14:53:20 +0200
Subject: [PATCH] Cell editor: the caret contrasts with the cell's fill (fixes
 #1408)

---
 crates/ui-egui/src/grid.rs                |  9 ++++
 crates/ui-egui/tests/cell_editor_caret.rs | 59 +++++++++++++++++++++++
 2 files changed, 68 insertions(+)
 create mode 100644 crates/ui-egui/tests/cell_editor_caret.rs

diff --git a/crates/ui-egui/src/grid.rs b/crates/ui-egui/src/grid.rs
index 00e0c18be..73d912907 100644
--- a/crates/ui-egui/src/grid.rs
+++ b/crates/ui-egui/src/grid.rs
@@ -2725,6 +2725,9 @@ fn in_cell_editor(app: &mut SheetApp, ui: &mut egui::Ui, geo: &Geo, sh: &Sheet,
     let id = egui::Id::new("gridcraft.cell_editor");
     let mut child = ui.new_child(egui::UiBuilder::new().max_rect(erect.shrink2(vec2(2.0, 1.0))));
     child.set_clip_rect(clip);
+    // The theme's caret is meant for the UI chrome and vanishes on some fills (and, in the dark
+    // theme, on the white paper): pick black or white against the fill, like Excel.
+    child.visuals_mut().text_cursor.stroke.color = ink_on(fill);
     crate::formula_bar::editor_widget(app, &mut child, id, font, false, erect.width() - 4.0);
 }
 
@@ -2734,6 +2737,12 @@ fn popup_dismissed(response: &egui::Response, was_open: bool) -> bool {
     response.ctx.input(|i| !i.focused || i.key_pressed(egui::Key::Escape)) || (was_open && response.clicked_elsewhere())
 }
 
+/// Black or white, whichever reads on `fill` (the threshold chart labels use).
+fn ink_on(fill: Color32) -> Color32 {
+    let l = 0.299 * fill.r() as f32 + 0.587 * fill.g() as f32 + 0.114 * fill.b() as f32;
+    if l < 150.0 { Color32::WHITE } else { Color32::BLACK }
+}
+
 /// The table whose header cell (`row`, `col`) was clicked, not just the first table in the column.
 fn filter_table_range(sh: &Sheet, col: u32, row: u32) -> Option<RangeRef> {
     sh.tables.iter().find(|t| t.range.start.row == row && col >= t.range.start.col && col <= t.range.end.col).map(|t| t.range)
diff --git a/crates/ui-egui/tests/cell_editor_caret.rs b/crates/ui-egui/tests/cell_editor_caret.rs
new file mode 100644
index 000000000..95b0ff6f3
--- /dev/null
+++ b/crates/ui-egui/tests/cell_editor_caret.rs
@@ -0,0 +1,59 @@
+//! The caret of the in-cell editor stays visible on the cell's fill, whatever the theme.
+use egui::{Color32, Context, RawInput, Rect, Shape, pos2, vec2};
+use gridcraft_engine::Session;
+use gridcraft_ui_egui::{SheetApp, theme};
+use serde_json::json;
+
+fn frame(ctx: &Context, app: &mut SheetApp) -> egui::FullOutput {
+    let mut output =
+        ctx.run_ui(RawInput { screen_rect: Some(Rect::from_min_size(pos2(0.0, 0.0), vec2(1200.0, 800.0))), ..Default::default() }, |ui| {
+            let ctx = ui.ctx().clone();
+            app.logic(&ctx);
+            app.ui(ui);
+        });
+    output.textures_delta.clear();
+    output
+}
+
+/// The caret painted while editing A1 over `fill` (`None`: no fill), with the app in `dark` mode.
+fn caret(fill: Option<&str>, dark: bool) -> Color32 {
+    let ctx = Context::default();
+    theme::apply(&ctx, dark);
+    let mut session = Session::new();
+    session.new_workbook();
+    let mut app = SheetApp::new(session, Default::default());
+    if let Some(color) = fill {
+        app.run("home.fillColor", json!({"range": "A1", "color": color})).unwrap();
+    }
+    for _ in 0..4 {
+        frame(&ctx, &mut app);
+    }
+    app.begin_edit(Some("abc".into()), false);
+    for _ in 0..4 {
+        frame(&ctx, &mut app);
+    }
+    let output = frame(&ctx, &mut app);
+    // The editor paints its backdrop first; the caret is the vertical segment after it.
+    let paper = fill.map_or(Color32::WHITE, |c| Color32::from_hex(c).unwrap());
+    let backdrop = output
+        .shapes
+        .iter()
+        .rposition(|s| matches!(&s.shape, Shape::Rect(r) if r.fill == paper && r.rect.width() > 20.0))
+        .expect("editor backdrop painted");
+    output.shapes[backdrop..]
+        .iter()
+        .find_map(|s| match &s.shape {
+            Shape::LineSegment { points, stroke } if points[0].x == points[1].x => Some(stroke.color),
+            _ => None,
+        })
+        .expect("caret painted")
+}
+
+#[test]
+fn caret_contrasts_with_the_cell_fill() {
+    assert_eq!(caret(Some("#1F3864"), false), Color32::WHITE, "dark fill, light theme");
+    assert_eq!(caret(Some("#1F3864"), true), Color32::WHITE, "dark fill, dark theme");
+    assert_eq!(caret(Some("#FFFF00"), true), Color32::BLACK, "light fill, dark theme");
+    assert_eq!(caret(None, true), Color32::BLACK, "no fill, dark theme: the cell is still white paper");
+    assert_eq!(caret(None, false), Color32::BLACK, "no fill, light theme");
+}