motion, scrolling and redraw are flat in file size now

zig build perf drives the core directly — event, effects, one frame, no pty —
over four generated fixtures: 1k lines, 50k, 300k, and 400 lines of 8000
columns, because a file that is long and a file that is wide fail differently.
Every sample seeks somewhere else in the file first, since measuring at line 3
of a 300k-line file hides exactly the bug.

perf record said half the run was scanning for newlines from byte 0. So File
carries a line index, built on demand and invalidated in exactly ONE place —
setContent, the funnel every content swap already goes through. That killed the
scrollbar's per-frame line count (12.6% of the whole run by itself), scrollBy,
ensureCursorVisible, lastNavRow, the syntax window bounds and two O(scroll)
walks. normalKey computed max_line as a const at the top: two full passes over
the buffer on every keystroke of every kind, for three g/G branches. It is lazy
now. The modal primitives each walked the text twice for the same line.

And the visible window was re-parsed on every scrolled row — a third of a
megabyte per keypress on the wide fixture. The highlighted range is remembered,
a scroll inside it is free, and only a re-parse that FOLLOWS a scroll takes
slack: doing it unconditionally made typing 2.1x slower, since every character
paid for a band it could never amortise.

One j on a 19 MB file: 37.8ms -> 266us. Render: 4.1ms -> 77us. Open costs 1.25x
more for the one extra pass, which buys 54x on every frame after, and 8 bytes
per line of memory.

Left standing, measured and named: edit-char is 14ms on 19MB because content is
immutable and every keystroke copies the buffer. A third of that is the index
rebuild, which could be a shift if setContent knew the edit offset; the rest
wants a rope. bodyText's double copy and Surface.print's per-cell decode never
rose above 2% of the profile afterwards, so they were left alone.

No golden moved.

1 files changed, 16 insertions, 5 deletions

diff --git a/src/modal.zig b/src/modal.zig
index 742dbc6f..cdc49142 100644
--- a/src/modal.zig
+++ b/src/modal.zig
@@ -438,7 +438,7 @@ pub fn lineStartOffset(content: []const u8, row: usize) usize {
     var off: usize = 0;
     var r: usize = 0;
     while (r < row) : (r += 1) {
-        const nl = std.mem.indexOfPos(u8, content, off, "\n") orelse return content.len;
+        const nl = std.mem.indexOfScalarPos(u8, content, off, '\n') orelse return content.len;
         off = nl + 1;
     }
     return off;
@@ -775,15 +775,24 @@ pub fn hxLineEndIdx(text: []const u8, line: usize) usize {
 
 /// gap offset -> (row, col) cell
 pub fn hxPos(text: []const u8, off: usize) Cursor {
-    const row = hxLineOf(text, off);
-    return .{ .row = row, .col = @min(off, text.len) - lineStartOffset(text, row) };
+    const o = @min(off, text.len);
+    // The line start is the byte after the last '\n' BEFORE off, which is the
+    // same number lineStartOffset(text, row) walks the whole prefix to reach —
+    // one backward scan of a single line instead of a second pass over
+    // everything above the cursor. On a multi-MB buffer that second pass was
+    // most of what a keystroke cost.
+    const s = if (std.mem.lastIndexOfScalar(u8, text[0..o], '\n')) |nl| nl + 1 else 0;
+    return .{ .row = hxLineOf(text, off), .col = o - s };
 }
 
 /// (row, col) -> clamped gap offset; col == line length lands ON the '\n'
 pub fn hxOff(text: []const u8, c: Cursor) usize {
     const row = @min(c.row, hxLineCount(text) - 1);
     const s = lineStartOffset(text, row);
-    return @min(s + c.col, hxLineEndIdx(text, row));
+    // hxLineEndIdx(text, row) inlined: it starts by walking to `row` again,
+    // and we are already standing there
+    const e = std.mem.indexOfScalarPos(u8, text, s, '\n') orelse text.len;
+    return @min(s + c.col, e);
 }
 
 pub const WordTarget = enum {
@@ -979,7 +988,9 @@ pub fn hxVertTarget(text: []const u8, pos: usize, down: bool, count: usize, goal
     const line = hxLineOf(text, pos);
     const nline = if (down) @min(line + @max(1, count), nlines - 1) else line -| @max(1, count);
     const s = lineStartOffset(text, nline);
-    return @min(s + goal_col, hxLineEndIdx(text, nline));
+    // hxLineEndIdx(text, nline) without its second walk to nline (see hxOff)
+    const e = std.mem.indexOfScalarPos(u8, text, s, '\n') orelse text.len;
+    return @min(s + goal_col, e);
 }
 
 /// f/F/t/T target cell. helix find_char: the exclusive (till) search starts