A selection deep in a long line costs a render what one at its start does: 5 MB in, /screen goes from 14.7 s to 5 ms

Every render measured a selection's and the cursor's columns from the
start of their line (lineDisplayOffset walked the whole prefix twice),
so a selection near the end of a 5 MB line stalled each frame, 3 s at
1 MB in and 14.7 s at 5 MB. The cost grew with the offset, with Wrap on
or off. Three changes make it independent of where the selection is:
- display width adds (a tab is tab_width wherever it stands), so the
  offset between two columns is the width of the text between them,
  never the prefix;
- the selection's rows are clamped to what is shown: a row wholly
  before the selection is skipped, a start before the row's first
  column starts there, and an end past its last column ends there;
- a head or cursor whose column lies past its visible row (the last
  visible row of a line owns every column after its start) is not
  measured, since it is not drawn.
Measured on a Debug build, 5 MB line: offset 10, 1 MB and 5 MB now take
11, 6 and 5 ms per /screen. A new perf gate case, deep-sel, renders with
the selection 2 MB into a line: 83 us on Debug. The baselines were
re-recorded for the new harness.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

1 files changed, 25 insertions, 3 deletions

diff --git a/test/perf.zig b/test/perf.zig
index 95b55748..94076a25 100644
--- a/test/perf.zig
+++ b/test/perf.zig
@@ -53,6 +53,10 @@ const Op = enum {
     /// close, as a shell's `>>` loop makes: each costs its own bytes, not a
     /// pass over the whole body for its cursor, line index and saved hash.
     body_appends,
+    /// A render with the selection 2 MB into one line: only the columns
+    /// shown are measured, so it costs what a selection at the line's
+    /// start does (4.5 s a frame at 5 MB before).
+    deep_selection,
 
     fn label(o: Op) []const u8 {
         return switch (o) {
@@ -65,6 +69,7 @@ const Op = enum {
             .edit_char => "edit-char",
             .body_write => "body-2m",
             .body_appends => "body-appends",
+            .deep_selection => "deep-sel",
         };
     }
 };
@@ -83,6 +88,16 @@ fn bodyWrite(core: *pardes.Pardes, serial: u32) void {
     _ = core.serveFs(.{ .tag = 4, .op = .release, .node = body, .handle = h, .opened = true });
 }
 
+/// One line of 2 MB appended to the file, no newline in it.
+fn longLine(core: *pardes.Pardes, serial: u32) void {
+    const Node = pardes.ctlfs.Node;
+    const body = Node.of(serial, .body);
+    @memset(&body_chunk, 'z');
+    const h = core.serveFs(.{ .tag = 1, .op = .open, .node = body, .omode = 1 }).handle;
+    for (0..8) |_| _ = core.serveFs(.{ .tag = 2, .op = .write, .node = body, .handle = h, .data = &body_chunk });
+    _ = core.serveFs(.{ .tag = 4, .op = .release, .node = body, .handle = h, .opened = true });
+}
+
 const append_chunk_len = 8 * 1024;
 const append_count = 64;
 
@@ -534,9 +549,16 @@ fn measure(op: Op, fx: Fixture, path: []const u8, expected_bytes: usize, reps: u
         core.update(.{ .key = .{ .cp = 'i', .text = "i" } });
         pump(core);
     }
+    if (op == .deep_selection) {
+        longLine(core, pane.serial);
+        pump(core);
+        const len: u32 = @intCast(pane.file.?.content.len);
+        pardes.ctlfs.pane.setDot(pane, .{ .q0 = len - 10, .q1 = len - 5 });
+        pump(core);
+    }
 
     for (0..warmup + reps) |n| {
-        seek(pane, fx, n);
+        if (op != .deep_selection) seek(pane, fx, n);
         _ = try frame(core); // settle the view at the new spot, untimed
         const before = .{ .row = pane.body.cur_row, .col = pane.body.cur_col, .scroll = pane.scroll(), .len = pane.file.?.content.len };
         const rect = core.rects[id];
@@ -545,7 +567,7 @@ fn measure(op: Op, fx: Fixture, path: []const u8, expected_bytes: usize, reps: u
         const t0 = nowNs();
         switch (op) {
             .open => unreachable,
-            .render => {},
+            .render, .deep_selection => {},
             .key_down => core.update(.{ .key = .{ .cp = 'j', .text = "j" } }),
             .key_right => core.update(.{ .key = .{ .cp = 'l', .text = "l" } }),
             .page_down => core.update(.{ .key = .{ .cp = pardes.Key.page_down } }),
@@ -561,7 +583,7 @@ fn measure(op: Op, fx: Fixture, path: []const u8, expected_bytes: usize, reps: u
             .open => unreachable,
             .body_write => if (pane.file.?.content.len != before.len + 8 * body_chunk.len) return error.BodyWriteWasNotAppended,
             .body_appends => if (pane.file.?.content.len != before.len + append_count * append_chunk_len) return error.BodyAppendsWereNotAppended,
-            .render => if (pane.body.cur_row != before.row or pane.body.cur_col != before.col or pane.file.?.content.len != before.len)
+            .render, .deep_selection => if (pane.body.cur_row != before.row or pane.body.cur_col != before.col or pane.file.?.content.len != before.len)
                 return error.RenderChangedDocument,
             .key_down => if (pane.body.cur_row != before.row + 1) return error.DownDidNotMove,
             .key_right => if (pane.body.cur_row != before.row or pane.body.cur_col != before.col + 1) return error.RightDidNotMove,