From 28cb3eacfcf1217b4d014dd864005deb2d7f0585 Mon Sep 17 00:00:00 2001
From: t <t@tjp.lol>
Date: Tue, 16 Jun 2026 11:49:54 -0600
Subject: tool collapsed state, and fixing dirty component flags

---
 src/tui_engine.zig | 63 ++++++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 52 insertions(+), 11 deletions(-)

(limited to 'src/tui_engine.zig')

diff --git a/src/tui_engine.zig b/src/tui_engine.zig
index 6c5b372..f63e269 100644
--- a/src/tui_engine.zig
+++ b/src/tui_engine.zig
@@ -485,10 +485,12 @@ pub const Engine = struct {
         var cut: ?usize = null;
 
         for (self.slots.items, 0..) |*slot, idx| {
-            const first_changed = slot.comp.firstLineChanged();
+            const dirty_signal = slot.comp.firstLineChanged();
             // Render only when there's a reason to: dirty signal, or first
-            // paint (no cached lines yet), or a forced full redraw.
-            const must_render = first_changed != null or slot.lines.len == 0 or self.force_full;
+            // paint (no cached lines yet), or a forced full redraw. The signal
+            // is ONLY a dirty/render hint; the repaint cut below is computed
+            // from the actual old-vs-new line diff after rendering.
+            const must_render = dirty_signal != null or slot.lines.len == 0 or self.force_full;
 
             var new_lines: []const []const u8 = undefined;
             if (must_render) {
@@ -503,6 +505,7 @@ pub const Engine = struct {
                 if (visibleWidth(line) > self.width) return Error.LineOverflow;
             }
 
+            const first_changed = firstDiffLines(slot.lines, new_lines);
             frames[idx] = .{
                 .new_lines = new_lines,
                 .first_changed = first_changed,
@@ -510,18 +513,13 @@ pub const Engine = struct {
                 .offset = offset,
             };
 
-            // 2. cut = min across ALL components (not just the first dirty).
+            // 2. cut = min across ALL components with an ACTUAL line diff.
+            // A dirty component whose re-rendered bytes are identical leaves
+            // first_changed null and does not move the repaint point.
             if (first_changed) |fc| {
                 const global = offset + fc;
                 cut = if (cut) |c| @min(c, global) else global;
             }
-            // A length delta is itself a change point even when the component
-            // reports firstLineChanged == null (the diff backstop will catch
-            // the content, but the boundary must roll the cut back).
-            if (new_lines.len != slot.line_count) {
-                const boundary = offset + @min(new_lines.len, slot.line_count);
-                cut = if (cut) |c| @min(c, boundary) else boundary;
-            }
 
             offset += new_lines.len;
         }
@@ -634,6 +632,21 @@ pub const Engine = struct {
         slot.lines = copies;
     }
 
+    /// Lowest local line index where `new_lines` differs from `old_lines`, or
+    /// null when they are byte-identical (including line count). This is the
+    /// authoritative repaint signal for a rendered component in the current
+    /// frame; pre-render `firstLineChanged()` is treated only as a cheap dirty
+    /// hint for whether the component needs to render.
+    fn firstDiffLines(old_lines: []const []const u8, new_lines: []const []const u8) ?usize {
+        const n = @min(old_lines.len, new_lines.len);
+        var i: usize = 0;
+        while (i < n) : (i += 1) {
+            if (!std.mem.eql(u8, old_lines[i], new_lines[i])) return i;
+        }
+        if (old_lines.len != new_lines.len) return n;
+        return null;
+    }
+
     /// Line-diff backstop (the CORRECTNESS FLOOR). Flattens the previous
     /// baseline lines (`slot.lines`) and the new frame lines into global arrays
     /// and returns the FIRST global index where they actually differ, or null
@@ -1324,6 +1337,34 @@ test "line-diff backstop corrects an inaccurate firstLineChanged" {
     try testing.expect(std.mem.indexOf(u8, out, terminal.seq.full_clear) == null);
 }
 
+test "dirty hint at 0 still repaints from actual first changed line" {
+    var buf = std.Io.Writer.Allocating.init(testing.allocator);
+    defer buf.deinit();
+    var eng = makeEngine(&buf, 80, 100);
+    defer eng.deinit();
+
+    // Simulates a component that conservatively reports dirty from its top
+    // (e.g. markDirty dropped its internal cache), while the engine still has
+    // the previous slot baseline and can cheaply find the true line diff.
+    var body = FakeComponent{
+        .scripts = &.{ &.{ "l0", "l1", "l2", "l3" }, &.{ "l0", "l1", "l2", "L3" } },
+        .first_changed = &.{ 0, 0 },
+    };
+    try eng.addComponent(body.comp());
+
+    try eng.render();
+    body.advance();
+    buf.clearRetainingCapacity();
+    try eng.render();
+
+    const out = buf.written();
+    try testing.expect(std.mem.indexOf(u8, out, "L3") != null);
+    try testing.expect(std.mem.indexOf(u8, out, "l0") == null);
+    try testing.expect(std.mem.indexOf(u8, out, "l1") == null);
+    try testing.expect(std.mem.indexOf(u8, out, "l2") == null);
+    try testing.expect(std.mem.indexOf(u8, out, terminal.seq.full_clear) == null);
+}
+
 test "consecutive differential frames move the cursor up correctly (streaming append)" {
     // Regression: the differential path must account for the cursor resting at
     // the END of the last content line (no trailing newline), not one row
-- 
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