From abbe69374ee40a8a20f6293725eb53aa4034d22d Mon Sep 17 00:00:00 2001
From: l0ng-ai <24760907+l0ng-ai@users.noreply.github.com>
Date: Mon, 7 Sep 2026 21:58:13 +0800
Subject: [PATCH] fix(diff-overlay): let the scrollbar reach the end of the
 patch
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

A list counts an item it has not laid out yet as zero tall, so the flattened
row list reported a patch of any length as about one viewport of content. The
scrollbar reads that height: its thumb filled the track, and dragging it from
the top to the bottom travelled 248px into an 800-line diff and stopped —
`scrollbar_drag_start_height` freezes the height for the duration of a drag,
so the gesture could not even grow into the rows it uncovered. The overlay
had a bar that could reach anywhere before the rows were virtualised.

`measure_all` would settle it by laying out every row on the first frame,
which is the cost the row list exists to avoid. Count the rows below the fold
at `DIFF_LINE_H` instead — the 19px both views already give a line of a patch
— through `ListState::with_size_hint`, a new gpui fork patch. File and hunk
rows are a few pixels taller, so the estimate runs short by a few percent
until they are measured; a diff is overwhelmingly its lines.

`a_long_patch_builds_only_the_rows_on_screen` now also asserts the bar's
reach. Without the hint it measures 248.5px against 802 rows.
---
 Cargo.lock             | 90 +++++++++++++++++++++---------------------
 Cargo.toml             | 10 ++++-
 src/ui/diff_overlay.rs | 53 +++++++++++++++++++------
 3 files changed, 95 insertions(+), 58 deletions(-)

diff --git a/Cargo.lock b/Cargo.lock
index 7974f7e1..69246c45 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -298,7 +298,7 @@ version = "1.1.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "40c48f72fd53cd289104fc64099abca73db4166ad86ea0b4341abe65af83dadc"
 dependencies = [
- "windows-sys 0.61.2",
+ "windows-sys 0.60.2",
 ]
 
 [[package]]
@@ -309,7 +309,7 @@ checksum = "291e6a250ff86cd4a820112fb8898808a366d8f9f58ce16d1f538353ad55747d"
 dependencies = [
  "anstyle",
  "once_cell_polyfill",
- "windows-sys 0.61.2",
+ "windows-sys 0.60.2",
 ]
 
 [[package]]
@@ -891,7 +891,7 @@ dependencies = [
  "bitflags 2.13.1",
  "cexpr",
  "clang-sys",
- "itertools 0.13.0",
+ "itertools 0.11.0",
  "log",
  "prettyplease",
  "proc-macro2",
@@ -1483,7 +1483,7 @@ dependencies = [
 [[package]]
 name = "collections"
 version = "0.1.0"
-source = "git+https://github.com/l0ng-ai/zed?branch=tty7#d99a40a5f79a173465feec00363d480f75881a81"
+source = "git+https://github.com/l0ng-ai/zed?branch=tty7#ece710e365575bc08656c4a8d20488769b2de9a6"
 dependencies = [
  "gpui_util",
  "indexmap",
@@ -2047,7 +2047,7 @@ dependencies = [
 [[package]]
 name = "derive_refineable"
 version = "0.1.0"
-source = "git+https://github.com/l0ng-ai/zed?branch=tty7#d99a40a5f79a173465feec00363d480f75881a81"
+source = "git+https://github.com/l0ng-ai/zed?branch=tty7#ece710e365575bc08656c4a8d20488769b2de9a6"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -2104,7 +2104,7 @@ dependencies = [
  "libc",
  "option-ext",
  "redox_users",
- "windows-sys 0.61.2",
+ "windows-sys 0.60.2",
 ]
 
 [[package]]
@@ -2142,7 +2142,7 @@ version = "0.5.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ab8ecd87370524b461f8557c119c405552c396ed91fc0a8eec68679eab26f94a"
 dependencies = [
- "libloading 0.8.9",
+ "libloading 0.7.4",
 ]
 
 [[package]]
@@ -2394,7 +2394,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "39cab71617ae0d63f51a36d69f866391735b51691dbda63cf6f96d042b63efeb"
 dependencies = [
  "libc",
- "windows-sys 0.61.2",
+ "windows-sys 0.60.2",
 ]
 
 [[package]]
@@ -3199,7 +3199,7 @@ dependencies = [
  "log",
  "presser",
  "thiserror 2.0.20",
- "windows 0.62.2",
+ "windows 0.58.0",
 ]
 
 [[package]]
@@ -3225,7 +3225,7 @@ dependencies = [
 [[package]]
 name = "gpui"
 version = "0.2.2"
-source = "git+https://github.com/l0ng-ai/zed?branch=tty7#d99a40a5f79a173465feec00363d480f75881a81"
+source = "git+https://github.com/l0ng-ai/zed?branch=tty7#ece710e365575bc08656c4a8d20488769b2de9a6"
 dependencies = [
  "accesskit",
  "anyhow",
@@ -3416,7 +3416,7 @@ dependencies = [
 [[package]]
 name = "gpui_linux"
 version = "0.1.0"
-source = "git+https://github.com/l0ng-ai/zed?branch=tty7#d99a40a5f79a173465feec00363d480f75881a81"
+source = "git+https://github.com/l0ng-ai/zed?branch=tty7#ece710e365575bc08656c4a8d20488769b2de9a6"
 dependencies = [
  "accesskit",
  "accesskit_unix",
@@ -3467,7 +3467,7 @@ dependencies = [
 [[package]]
 name = "gpui_macos"
 version = "0.1.0"
-source = "git+https://github.com/l0ng-ai/zed?branch=tty7#d99a40a5f79a173465feec00363d480f75881a81"
+source = "git+https://github.com/l0ng-ai/zed?branch=tty7#ece710e365575bc08656c4a8d20488769b2de9a6"
 dependencies = [
  "accesskit",
  "accesskit_macos",
@@ -3514,7 +3514,7 @@ dependencies = [
 [[package]]
 name = "gpui_macros"
 version = "0.1.0"
-source = "git+https://github.com/l0ng-ai/zed?branch=tty7#d99a40a5f79a173465feec00363d480f75881a81"
+source = "git+https://github.com/l0ng-ai/zed?branch=tty7#ece710e365575bc08656c4a8d20488769b2de9a6"
 dependencies = [
  "heck 0.5.0",
  "proc-macro2",
@@ -3525,7 +3525,7 @@ dependencies = [
 [[package]]
 name = "gpui_platform"
 version = "0.1.0"
-source = "git+https://github.com/l0ng-ai/zed?branch=tty7#d99a40a5f79a173465feec00363d480f75881a81"
+source = "git+https://github.com/l0ng-ai/zed?branch=tty7#ece710e365575bc08656c4a8d20488769b2de9a6"
 dependencies = [
  "console_error_panic_hook",
  "gpui",
@@ -3538,7 +3538,7 @@ dependencies = [
 [[package]]
 name = "gpui_shared_string"
 version = "0.1.0"
-source = "git+https://github.com/l0ng-ai/zed?branch=tty7#d99a40a5f79a173465feec00363d480f75881a81"
+source = "git+https://github.com/l0ng-ai/zed?branch=tty7#ece710e365575bc08656c4a8d20488769b2de9a6"
 dependencies = [
  "schemars",
  "serde",
@@ -3548,7 +3548,7 @@ dependencies = [
 [[package]]
 name = "gpui_util"
 version = "0.1.0"
-source = "git+https://github.com/l0ng-ai/zed?branch=tty7#d99a40a5f79a173465feec00363d480f75881a81"
+source = "git+https://github.com/l0ng-ai/zed?branch=tty7#ece710e365575bc08656c4a8d20488769b2de9a6"
 dependencies = [
  "anyhow",
  "log",
@@ -3557,7 +3557,7 @@ dependencies = [
 [[package]]
 name = "gpui_web"
 version = "0.1.0"
-source = "git+https://github.com/l0ng-ai/zed?branch=tty7#d99a40a5f79a173465feec00363d480f75881a81"
+source = "git+https://github.com/l0ng-ai/zed?branch=tty7#ece710e365575bc08656c4a8d20488769b2de9a6"
 dependencies = [
  "anyhow",
  "console_error_panic_hook",
@@ -3581,7 +3581,7 @@ dependencies = [
 [[package]]
 name = "gpui_wgpu"
 version = "0.1.0"
-source = "git+https://github.com/l0ng-ai/zed?branch=tty7#d99a40a5f79a173465feec00363d480f75881a81"
+source = "git+https://github.com/l0ng-ai/zed?branch=tty7#ece710e365575bc08656c4a8d20488769b2de9a6"
 dependencies = [
  "anyhow",
  "bytemuck",
@@ -3610,7 +3610,7 @@ dependencies = [
 [[package]]
 name = "gpui_windows"
 version = "0.1.0"
-source = "git+https://github.com/l0ng-ai/zed?branch=tty7#d99a40a5f79a173465feec00363d480f75881a81"
+source = "git+https://github.com/l0ng-ai/zed?branch=tty7#ece710e365575bc08656c4a8d20488769b2de9a6"
 dependencies = [
  "accesskit",
  "accesskit_windows",
@@ -3939,7 +3939,7 @@ dependencies = [
 [[package]]
 name = "http_client"
 version = "0.1.0"
-source = "git+https://github.com/l0ng-ai/zed?branch=tty7#d99a40a5f79a173465feec00363d480f75881a81"
+source = "git+https://github.com/l0ng-ai/zed?branch=tty7#ece710e365575bc08656c4a8d20488769b2de9a6"
 dependencies = [
  "anyhow",
  "async-compression",
@@ -3964,7 +3964,7 @@ dependencies = [
 [[package]]
 name = "http_client_tls"
 version = "0.1.0"
-source = "git+https://github.com/l0ng-ai/zed?branch=tty7#d99a40a5f79a173465feec00363d480f75881a81"
+source = "git+https://github.com/l0ng-ai/zed?branch=tty7#ece710e365575bc08656c4a8d20488769b2de9a6"
 dependencies = [
  "rustls",
  "rustls-platform-verifier",
@@ -4057,7 +4057,7 @@ dependencies = [
  "js-sys",
  "log",
  "wasm-bindgen",
- "windows-core 0.62.2",
+ "windows-core 0.58.0",
 ]
 
 [[package]]
@@ -5092,7 +5092,7 @@ checksum = "7ebb8d8732c6a6df3d8f032a82911cfc747e00efb95cc46e8d0acd5b5b88570c"
 [[package]]
 name = "media"
 version = "0.1.0"
-source = "git+https://github.com/l0ng-ai/zed?branch=tty7#d99a40a5f79a173465feec00363d480f75881a81"
+source = "git+https://github.com/l0ng-ai/zed?branch=tty7#ece710e365575bc08656c4a8d20488769b2de9a6"
 dependencies = [
  "anyhow",
  "bindgen",
@@ -5202,7 +5202,7 @@ version = "0.6.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "536bfad37a309d62069485248eeaba1e8d9853aaf951caaeaed0585a95346f08"
 dependencies = [
- "windows-sys 0.61.2",
+ "windows-sys 0.60.2",
 ]
 
 [[package]]
@@ -5258,7 +5258,7 @@ dependencies = [
  "once_cell",
  "png",
  "thiserror 2.0.20",
- "windows-sys 0.61.2",
+ "windows-sys 0.60.2",
 ]
 
 [[package]]
@@ -5481,7 +5481,7 @@ version = "0.50.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7957b9740744892f114936ab4a57b3f487491bbeafaf8083688b16841a4240e5"
 dependencies = [
- "windows-sys 0.61.2",
+ "windows-sys 0.60.2",
 ]
 
 [[package]]
@@ -6204,7 +6204,7 @@ checksum = "9b4f627cb1b25917193a259e49bdad08f671f8d9708acfd5fe0a8c1455d87220"
 [[package]]
 name = "perf"
 version = "0.1.0"
-source = "git+https://github.com/l0ng-ai/zed?branch=tty7#d99a40a5f79a173465feec00363d480f75881a81"
+source = "git+https://github.com/l0ng-ai/zed?branch=tty7#ece710e365575bc08656c4a8d20488769b2de9a6"
 dependencies = [
  "collections",
  "serde",
@@ -6900,7 +6900,7 @@ dependencies = [
  "once_cell",
  "socket2",
  "tracing",
- "windows-sys 0.61.2",
+ "windows-sys 0.60.2",
 ]
 
 [[package]]
@@ -7207,7 +7207,7 @@ dependencies = [
 [[package]]
 name = "refineable"
 version = "0.1.0"
-source = "git+https://github.com/l0ng-ai/zed?branch=tty7#d99a40a5f79a173465feec00363d480f75881a81"
+source = "git+https://github.com/l0ng-ai/zed?branch=tty7#ece710e365575bc08656c4a8d20488769b2de9a6"
 dependencies = [
  "derive_refineable",
 ]
@@ -7250,7 +7250,7 @@ checksum = "19b30a45b0cd0bcca8037f3d0dc3421eaf95327a17cad11964fb8179b4fc4832"
 [[package]]
 name = "reqwest_client"
 version = "0.1.0"
-source = "git+https://github.com/l0ng-ai/zed?branch=tty7#d99a40a5f79a173465feec00363d480f75881a81"
+source = "git+https://github.com/l0ng-ai/zed?branch=tty7#ece710e365575bc08656c4a8d20488769b2de9a6"
 dependencies = [
  "anyhow",
  "bytes",
@@ -7611,7 +7611,7 @@ dependencies = [
  "errno",
  "libc",
  "linux-raw-sys 0.12.1",
- "windows-sys 0.61.2",
+ "windows-sys 0.60.2",
 ]
 
 [[package]]
@@ -7784,7 +7784,7 @@ dependencies = [
 [[package]]
 name = "scheduler"
 version = "0.1.0"
-source = "git+https://github.com/l0ng-ai/zed?branch=tty7#d99a40a5f79a173465feec00363d480f75881a81"
+source = "git+https://github.com/l0ng-ai/zed?branch=tty7#ece710e365575bc08656c4a8d20488769b2de9a6"
 dependencies = [
  "async-task",
  "backtrace",
@@ -8389,7 +8389,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "c3d1e2c7f27f8d4cb10542a02c49005dbd6e93095799d6f3be745fae9f8fedd4"
 dependencies = [
  "libc",
- "windows-sys 0.61.2",
+ "windows-sys 0.60.2",
 ]
 
 [[package]]
@@ -8515,7 +8515,7 @@ dependencies = [
  "cfg-if",
  "libc",
  "psm",
- "windows-sys 0.61.2",
+ "windows-sys 0.60.2",
 ]
 
 [[package]]
@@ -8627,7 +8627,7 @@ checksum = "13c2bddecc57b384dee18652358fb23172facb8a2c51ccc10d74c157bdea3292"
 [[package]]
 name = "sum_tree"
 version = "0.1.0"
-source = "git+https://github.com/l0ng-ai/zed?branch=tty7#d99a40a5f79a173465feec00363d480f75881a81"
+source = "git+https://github.com/l0ng-ai/zed?branch=tty7#ece710e365575bc08656c4a8d20488769b2de9a6"
 dependencies = [
  "heapless",
  "log",
@@ -8917,7 +8917,7 @@ dependencies = [
  "getrandom 0.4.3",
  "once_cell",
  "rustix 1.1.4",
- "windows-sys 0.61.2",
+ "windows-sys 0.60.2",
 ]
 
 [[package]]
@@ -9384,7 +9384,7 @@ dependencies = [
  "once_cell",
  "png",
  "thiserror 2.0.20",
- "windows-sys 0.61.2",
+ "windows-sys 0.60.2",
 ]
 
 [[package]]
@@ -9896,7 +9896,7 @@ checksum = "f2f6fb2847f6742cd76af783a2a2c49e9375d0a111c7bef6f71cd9e738c72d6e"
 dependencies = [
  "memoffset 0.9.1",
  "tempfile",
- "windows-sys 0.61.2",
+ "windows-sys 0.60.2",
 ]
 
 [[package]]
@@ -10108,7 +10108,7 @@ checksum = "06abde3611657adf66d383f00b093d7faecc7fa57071cce2578660c9f1010821"
 [[package]]
 name = "util"
 version = "0.1.0"
-source = "git+https://github.com/l0ng-ai/zed?branch=tty7#d99a40a5f79a173465feec00363d480f75881a81"
+source = "git+https://github.com/l0ng-ai/zed?branch=tty7#ece710e365575bc08656c4a8d20488769b2de9a6"
 dependencies = [
  "anyhow",
  "async-fs",
@@ -10147,7 +10147,7 @@ dependencies = [
 [[package]]
 name = "util_macros"
 version = "0.1.0"
-source = "git+https://github.com/l0ng-ai/zed?branch=tty7#d99a40a5f79a173465feec00363d480f75881a81"
+source = "git+https://github.com/l0ng-ai/zed?branch=tty7#ece710e365575bc08656c4a8d20488769b2de9a6"
 dependencies = [
  "perf",
  "quote",
@@ -10740,7 +10740,7 @@ version = "0.1.11"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "c2a7b1c03c876122aa43f3020e6c3c3ee5c05081c9a00739faf7503aeba10d22"
 dependencies = [
- "windows-sys 0.61.2",
+ "windows-sys 0.48.0",
 ]
 
 [[package]]
@@ -11491,7 +11491,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7d6f32a0ff4a9f6f01231eb2059cc85479330739333e0e58cadf03b6af2cca10"
 dependencies = [
  "cfg-if",
- "windows-sys 0.61.2",
+ "windows-sys 0.60.2",
 ]
 
 [[package]]
@@ -12048,7 +12048,7 @@ dependencies = [
 [[package]]
 name = "zlog"
 version = "0.1.0"
-source = "git+https://github.com/l0ng-ai/zed?branch=tty7#d99a40a5f79a173465feec00363d480f75881a81"
+source = "git+https://github.com/l0ng-ai/zed?branch=tty7#ece710e365575bc08656c4a8d20488769b2de9a6"
 dependencies = [
  "anyhow",
  "chrono",
@@ -12093,7 +12093,7 @@ dependencies = [
 [[package]]
 name = "ztracing"
 version = "0.1.0"
-source = "git+https://github.com/l0ng-ai/zed?branch=tty7#d99a40a5f79a173465feec00363d480f75881a81"
+source = "git+https://github.com/l0ng-ai/zed?branch=tty7#ece710e365575bc08656c4a8d20488769b2de9a6"
 dependencies = [
  "tracing",
  "tracing-subscriber",
@@ -12104,7 +12104,7 @@ dependencies = [
 [[package]]
 name = "ztracing_macro"
 version = "0.1.0"
-source = "git+https://github.com/l0ng-ai/zed?branch=tty7#d99a40a5f79a173465feec00363d480f75881a81"
+source = "git+https://github.com/l0ng-ai/zed?branch=tty7#ece710e365575bc08656c4a8d20488769b2de9a6"
 
 [[package]]
 name = "zune-core"
diff --git a/Cargo.toml b/Cargo.toml
index 31d358e5..ac4ecab9 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -367,7 +367,7 @@ lto = "thin"
 codegen-units = 1
 
 # ---- gpui fork ------------------------------------------------------------
-# Our `tty7` branch (cut from the pinned upstream rev, three commits on top) carries:
+# Our `tty7` branch (cut from the pinned upstream rev) carries:
 #
 # 1. `prefers_ime_for_printable_keys` takes the keystroke, so an input handler can
 #    answer per key instead of per view. tty7 needs it for Option-as-Meta — macOS
@@ -386,6 +386,14 @@ codegen-units = 1
 # 3. resvg/usvg bumped 0.45 → 0.47 so gpui's SVG stack unifies with the resvg
 #    tty7 pins directly above (follow-up to the #227 dependabot bump).
 #
+# 4. `ListState::with_size_hint`. A list counts an item it has not laid out yet
+#    as zero tall, so a long document reports itself as about one viewport and
+#    a scrollbar reading that height drags one viewport and stops — the diff
+#    overlay scrolls a whole working tree through one of these. The hint counts
+#    the unmeasured items at an estimate instead, which `measure_all` would
+#    settle exactly at the price of laying out the whole document on the first
+#    frame (see `ui::diff_overlay::DIFF_LINE_H`).
+#
 # Patching by source rather than editing the `gpui`/`gpui_platform` pins above is
 # deliberate: `gpui-component` declares its own `gpui` from the upstream URL, and
 # a plain pin swap would put two incompatible copies of gpui in the tree. `[patch]`
diff --git a/src/ui/diff_overlay.rs b/src/ui/diff_overlay.rs
index 1639cce4..d3d60b72 100644
--- a/src/ui/diff_overlay.rs
+++ b/src/ui/diff_overlay.rs
@@ -168,7 +168,8 @@ impl Tty7App {
             focus,
             preview: None,
             preview_loading: None,
-            list: gpui::ListState::new(0, gpui::ListAlignment::Top, px(256.)),
+            list: gpui::ListState::new(0, gpui::ListAlignment::Top, px(256.))
+                .with_size_hint(DIFF_LINE_H),
             rows: Rc::new(Vec::new()),
             rows_key: None,
             epoch: None,
@@ -897,15 +898,14 @@ impl Tty7App {
         // padding here would be silently ignored. The rows carry their own —
         // see `diff_row_element`.
         .py_4();
-        // A whole working tree can scroll past here with nothing to say how
-        // far it runs or where in it you are — the one long document in the
-        // app without the bar every other scroll area has.
-        //
-        // The bar reads the list's own height, and a list only knows the rows
-        // it has measured: until the reader has been to the bottom once, the
-        // thumb is sized against a document that is still being discovered, so
-        // it shrinks as they scroll. `ListState::measure_all` would settle it
-        // by laying out every row on the first frame, which is the cost this
+        // The bar reads the list's own height, and a list only counts the
+        // rows it has measured. Left at that, a patch of any length would
+        // report itself as one screen long and the thumb would fill the
+        // track: a drag from top to bottom would travel one screen and stop,
+        // on the one document in the app long enough to need the bar. The
+        // rows below the fold are counted at `DIFF_LINE_H` until they are
+        // laid out — `ListState::measure_all` would settle it exactly, by
+        // laying out every row on the first frame, which is the cost this
         // whole list exists to avoid.
         crate::ui::scrollbar::with_vertical_scrollbar("diff-overlay-scrollbar", body, &list)
     }
@@ -1055,6 +1055,16 @@ const FILE_ROW_H: Pixels = px(26.);
 /// The radius on a row that lights up under the pointer. Matches the panel's.
 const ROW_RADIUS: Pixels = px(5.);
 
+/// The height of one line of a patch, in either view.
+///
+/// Also what the list counts a row it has not laid out yet at. A list knows
+/// only the rows it has measured, so without an estimate for the rest a patch
+/// of any length reports itself as one screen long — and the scrollbar, which
+/// reads that height, drags the reader one screen and stops. The file and hunk
+/// rows are a few pixels taller, so the estimate runs a little short until
+/// they have been measured; a diff is overwhelmingly its lines.
+const DIFF_LINE_H: Pixels = px(19.);
+
 /// The rule between one hunk and the last line of the one before it.
 ///
 /// Barely there on purpose: with the cards gone it is the only line left in
@@ -1322,7 +1332,7 @@ fn diff_untracked_row(
 fn diff_split_row(row: &SplitRow, font: &SharedString, cx: &gpui::App) -> impl IntoElement {
     h_flex()
         .w_full()
-        .h(px(19.))
+        .h(DIFF_LINE_H)
         .items_stretch()
         .text_xs()
         .font_family(font.clone())
@@ -1391,7 +1401,7 @@ fn diff_unified_row(row: &UnifiedRow, font: &SharedString, cx: &gpui::App) -> im
     };
     h_flex()
         .w_full()
-        .h(px(19.))
+        .h(DIFF_LINE_H)
         .items_center()
         .text_xs()
         .font_family(font.clone())
@@ -2778,6 +2788,25 @@ mod render_idle_gpui_tests {
             "a frame built {built} of {rows} rows: the list is not virtualised"
         );
 
+        // The other half of virtualising: a list counts the rows it has not
+        // laid out at zero unless it is given an estimate, and the scrollbar
+        // reads that count as the length of the document. Without the size
+        // hint the bar reaches 248px into this patch — its thumb fills the
+        // track, and dragging it to the bottom lands a screen down.
+        let reach = app.update_in(&mut vcx, |app, _, _| {
+            app.tabs[app.active]
+                .diff_overlay
+                .as_ref()
+                .expect("the overlay is open")
+                .list
+                .max_offset_for_scrollbar()
+                .y
+        });
+        assert!(
+            reach > DIFF_LINE_H * (rows as f32 * 0.75),
+            "the scrollbar reaches {reach} into a patch of {rows} rows"
+        );
+
         let _ = std::fs::remove_dir_all(&root);
     }