Prompt rail: one Codex-sized height that nothing changes; the wave keeps up with a sweep

webviews/src/agent-session/acpmux/threadMinimap/ThreadMinimap.tsx

@@ -402,21 +402,27 @@ export function ThreadMinimap({
   /// index (`earlier`) give every prompt its tick from the first frame, so the box is full height and
   /// centered once, and history paging in neither grows nor re-centres it (Leo, 2026-11-03: "mostly i
   /// just don't like it when the thing shifts and whatnot trying to center itself").
-  /// Never shorter than the prompts on screen, so the band always draws whole (model.ts railHeight).
+  /// One height for a fixed arm; the others are never shorter than the prompts on screen (railHeight).
+  const heightConfig = railHeightConfig(heightArm);
   const boxRail = railHeight(
     turns.length,
     windowHeight,
     boxHeight,
     boxFoot,
     onScreen.last - onScreen.first + 1,
-    railHeightConfig(heightArm),
+    heightConfig,
   );
-  const boxTop = railTopAt(boxRail, boxHeight, boxFoot);
   /// The bottom stack's top as it is now (VirtualTranscript `railBox.live`): the rail's foot and the
   /// card's both stay above it, so the two never disagree on where the view ends.
   const liveFoot = railBox?.live ?? boxFoot ?? viewportHeight;
-  /// The list's own height: the whole box, less what a stack grown since rest takes (railClearHeight).
-  const height = railClearHeight(boxRail, boxTop, boxHeight, boxFoot ?? boxHeight, liveFoot);
+  /// A fixed rail neither moves nor changes height when the stack grows (Leo, 2026-11-04: "the height
+  /// of the left scroll rail should never be affected by anything"; 2026-11-03: "i just don't like it
+  /// when the thing shifts"): a stack tall enough to reach it lays over its foot. The other arms keep
+  /// their top and shorten from the foot (railClearHeight).
+  const boxTop = railTopAt(boxRail, boxHeight, boxFoot);
+  const height = heightConfig.fixed
+    ? boxRail
+    : railClearHeight(boxRail, boxTop, boxHeight, boxFoot ?? boxHeight, liveFoot);
   /// Where the list's first row draws in the view.
   const railTop = boxTop;
   // The compact rail is pinned to the pane's left edge. It never follows the centered transcript or

webviews/src/agent-session/acpmux/threadMinimap/height.experiment.ts

@@ -2,15 +2,30 @@
 import { defineExperiment } from "../../../experiments/experiment";
 import { RAIL_HEIGHTS, type RailHeightArm } from "./model";
 
-/// The compact rail's initial and maximum height. Leo asked for a rail that grows toward ChatGPT's
-/// nearly full-pane rail instead of tracking the visible transcript slice. The gallery compares four
-/// choices; `initial-320-max-92` is the selected balance between a generous first view and room for the
-/// transcript's own edge mask. Dogfood switches from `cmux.experiments` with this experiment id.
+/// The compact rail's initial and maximum height. Leo first asked for a rail that grows toward
+/// ChatGPT's nearly full-pane rail instead of tracking the visible transcript slice
+/// (`initial-320-max-92` shipped for that). Then, 2026-11-04: "the left side session rail should just
+/// be having a nicer max height instead of going all the way up and down. make it look like codex's
+/// size." and "the height of the left scroll rail should never be affected by anything". The `fixed-*`
+/// arms are one height for every chat, whatever its prompt count or the pane's height; `fixed-240`
+/// ships. Dogfood switches from `cmux.experiments` with this experiment id.
 export const threadMinimapHeight = defineExperiment({
   id: "thread-minimap-height",
   title: "Thread minimap height",
   description: "How tall the compact prompt rail starts and how close its maximum gets to the pane height.",
   arms: {
+    "fixed-200": {
+      label: "Fixed 200px",
+      description: "One short height for every chat; longer chats scroll the rail's own list.",
+    },
+    "fixed-240": {
+      label: "Fixed 240px",
+      description: "The Codex rail's size: one height for every chat, unaffected by prompt count or pane height.",
+    },
+    "fixed-280": {
+      label: "Fixed 280px",
+      description: "One slightly taller height for every chat.",
+    },
     "initial-240-max-80": {
       label: "240px / 80%",
       description: "A modest initial rail, capped at 80% of the pane.",
@@ -28,7 +43,7 @@ export const threadMinimapHeight = defineExperiment({
       description: "The tallest initial rail, using all available cleared pane room.",
     },
   },
-  defaultArm: "initial-320-max-92",
+  defaultArm: "fixed-240",
 });
 
 export const railHeightConfig = (arm: RailHeightArm) => RAIL_HEIGHTS[arm];

webviews/src/agent-session/acpmux/threadMinimap/model.ts

@@ -79,16 +79,26 @@ export function railPlacement(
 }
 /// Height choices for the compact rail. The initial height keeps a short thread from looking like a
 /// transcript slice; the share and row cap let a long thread grow toward the pane's full height before
-/// its own list scrolls. The gallery compares these arms; the selected default is the middle ground.
-export type RailHeightConfig = { initial: number; maxRows: number; maxShare: number };
+/// its own list scrolls. The `fixed-*` arms are one height for every chat, the Codex rail's size: the
+/// prompt count, the window and the pane leave it alone, and past it the list scrolls (Leo,
+/// 2026-11-04: "the left side session rail should just be having a nicer max height instead of going
+/// all the way up and down. make it look like codex's size." and "the height of the left scroll rail
+/// should never be affected by anything"). Only a view too short to hold it still clips it
+/// (railRoom). `fixed-240` ships; the gallery compares the arms.
+/// A `fixed` arm is one height whatever the chat, window or bottom stack (Leo, 2026-11-04: "the height
+/// of the left scroll rail should never be affected by anything"): only a view too short for it clips it.
+export type RailHeightConfig = { initial: number; maxRows: number; maxShare: number; fixed?: boolean };
 export const RAIL_HEIGHTS = {
+  "fixed-200": { initial: 200, maxRows: 20, maxShare: 1, fixed: true },
+  "fixed-240": { initial: 240, maxRows: 24, maxShare: 1, fixed: true },
+  "fixed-280": { initial: 280, maxRows: 28, maxShare: 1, fixed: true },
   "initial-240-max-80": { initial: 240, maxRows: 96, maxShare: 0.8 },
   "initial-320-max-92": { initial: 320, maxRows: 128, maxShare: 0.92 },
   "initial-400-max-96": { initial: 400, maxRows: 160, maxShare: 0.96 },
   "initial-480-max-full": { initial: 480, maxRows: 200, maxShare: 1 },
 } as const satisfies Record<string, RailHeightConfig>;
 export type RailHeightArm = keyof typeof RAIL_HEIGHTS;
-export const DEFAULT_RAIL_HEIGHT: RailHeightArm = "initial-320-max-92";
+export const DEFAULT_RAIL_HEIGHT: RailHeightArm = "fixed-240";
 /// The fisheye by row distance from the hovered (or scrubbed) row: 26, 20, 14, 10, then 6 px.
 const PROGRESS = [1, 0.7, 0.4, 0.2] as const;
 /// Space kept free above and below the card inside the transcript's viewport.
@@ -196,11 +206,14 @@ export function waveFollow(focus: number, target: number, elapsed: number): numb
 /// ease to its new length when the hovered row changes, so a sweep zips down the rail tick by tick.
 /// A tick heading somewhere new starts from where it is, never from rest.
 export type ZipMotion = { from: number; to: number; start: number; ms: number };
-/// A tick that is not the hovered one easing to its new length.
+/// A tick shrinking to its new length as the hovered row moves away: the zipper's trailing tail.
 export const ZIP_MS = MOTION_MS.standard;
-/// The hovered tick growing as the pointer arrives from off the rail. On a move between rows the
-/// hovered tick is already longer than its old neighbours would allow it to wait, so it takes its
-/// full length at once (`zipStep`).
+/// A tick growing: the hovered tick as the pointer arrives from off the rail, and every tick the bump
+/// moves onto. Growing over the quick step keeps the bump's leading side up with the pointer: over
+/// ZIP_MS a sweep left the hovered tick long and the rows ahead of it short, holes in the wave (Leo,
+/// 2026-11-04: "hovering across the session rail gets a bit laggy and the missing chunks of the wave
+/// bother me :( it needs to get faster"). On a move between rows the hovered tick is already longer
+/// than its old neighbours would allow it to wait, so it takes its full length at once (`zipStep`).
 export const ZIP_HOVER_MS = MOTION_MS.quick;
 /// The pointer leaving: every tick shrinks back to rest over the scale's ceiling (Leo, 2026-11-03:
 /// "chatgpt has an animation for when it shrinks").
@@ -242,7 +255,7 @@ export function zipRetarget(
       motions.delete(index);
       continue;
     }
-    const ms = hovered === undefined ? ZIP_RELEASE_MS : index === hovered ? ZIP_HOVER_MS : ZIP_MS;
+    const ms = hovered === undefined ? ZIP_RELEASE_MS : index === hovered || to > value ? ZIP_HOVER_MS : ZIP_MS;
     motions.set(index, { from: value, to, start: now, ms });
   }
 }
@@ -293,8 +306,9 @@ export function railRoom(viewportHeight: number, foot = viewportHeight): { top:
 /// 70% of the window, after which the rows scroll. It fits the room the transcript gives it
 /// (railRoom: its viewport less the mask's bands), so a short pane's rows all scroll into view and no
 /// tick sits under the transcript's fades (Leo 2026-10-22: "it wouldn't show the highlighted bottom
-/// ... it just didn't go to the bottom"). It is never shorter than the band, the `band` prompts on
-/// screen: in a pane too short for both, the mask's clearance gives way first, up to the whole view.
+/// ... it just didn't go to the bottom"). A `fixed` arm is its one height, clipped only by that room.
+/// The others are never shorter than the band, the `band` prompts on screen: in a pane too short for
+/// both, the mask's clearance gives way first, up to the whole view.
 export function railHeight(
   count: number,
   windowHeight: number,
@@ -305,6 +319,11 @@ export function railHeight(
 ): number {
   if (count <= 0) return 0;
   const room = railRoom(viewportHeight, foot);
+  // A fixed rail does not grow for the band: its list scrolls to keep the band in view (railScrollTop).
+  if (config.fixed)
+    return viewportHeight > 0
+      ? Math.max(0, Math.min(config.initial, Math.floor(room.bottom - room.top)))
+      : config.initial;
   const height = Math.min(
     Math.max(count * ROW_PITCH, config.initial),
     config.maxRows * ROW_PITCH,

webviews/src/agent-session/acpmux/threadMinimap/threadMinimapHeight.gallery.ts

@@ -34,7 +34,7 @@ export default agentPaneEntry({
   },
   variants: {
     "eighteen-prompts": {
-      note: "Eighteen prompts with a tall initial rail. Compare how much quiet vertical rail each height arm gives the thread before overflow.",
+      note: "Eighteen prompts. The fixed arms (200, 240 shipped, 280) keep one height with quiet rail under the last tick; the older initial-* arms are here for comparison.",
       snapshot: chat(medium, { title: "Thread minimap height: initial rail" }),
       play: async (ctx) => {
         if (!ctx.document.querySelector('[data-tick="8"]')) return;
@@ -44,7 +44,7 @@ export default agentPaneEntry({
       },
     },
     "seventy-prompts": {
-      note: "Seventy prompts. Compare the maximum arms as they approach the cleared pane height and leave only the rail's own scroll for overflow.",
+      note: "Seventy prompts. A fixed arm stays at its one height and its list scrolls; the initial-* arms grow toward the pane before they scroll.",
       snapshot: chat(long, { title: "Thread minimap height: long chat" }),
       play: async (ctx) => {
         if (!ctx.document.querySelector('[data-tick="40"]')) return;
@@ -58,7 +58,7 @@ export default agentPaneEntry({
       },
     },
     "tall-pane-caps": {
-      note: "Two hundred sixty prompts in a tall pane. The four arms reach distinct row caps before the viewport clearance bound.",
+      note: "Two hundred sixty prompts in a 2100 px pane. A fixed arm keeps the same height as in a 720 px pane; the initial-* arms reach their row caps.",
       height: 2100,
       snapshot: chat(tall, { title: "Thread minimap height: distinct caps" }),
       play: async (ctx) => {

webviews/test/thread-minimap-rail.test.tsx

@@ -346,20 +346,27 @@ test("focus in the card keeps it; Escape there or focus leaving rail and card cl
 
 test("the rail never outgrows a short transcript", async () => {
   const { railHeight, RAIL_HEIGHTS } = await import("../src/agent-session/acpmux/threadMinimap/model");
-  // 60 prompts in an 820 px window over a 400 px transcript: the rail fits the transcript, less the
+  // 60 prompts in an 820 px window over a 200 px transcript: the rail fits the transcript, less the
   // mask's 18 px bands and 4 px past each.
-  expect(railHeight(60, 820, 400)).toBe(400 - 22 - 22);
-  expect(railHeight(60, 820, 820)).toBe(600);
+  expect(railHeight(60, 820, 200)).toBe(200 - 22 - 22);
   // Not measured yet: the window caps it alone.
   expect(railHeight(60, 820, 0)).toBe(railHeight(60, 820));
-  // The selected height arm starts a short thread tall and grows toward the cleared pane room.
-  expect(railHeight(4, 720, 720)).toBe(320);
-  expect(railHeight(70, 720, 720)).toBe(Math.floor(720 * 0.92));
   // The visible transcript band no longer controls the rail's initial height.
   expect(railHeight(18, 720, 720, 720, 1)).toBe(railHeight(18, 720, 720, 720, 4));
   expect(railHeight(18, 720, 720, 720, 4, RAIL_HEIGHTS["initial-240-max-80"])).toBe(240);
 });
 
+test("the rail is one height, the Codex rail's, whatever the prompt count or the pane's height", async () => {
+  const { railHeight } = await import("../src/agent-session/acpmux/threadMinimap/model");
+  // Leo, 2026-11-04: "make it look like codex's size" and "the height of the left scroll rail should
+  // never be affected by anything". Past 24 prompts the rail's own list scrolls.
+  for (const count of [1, 4, 12, 24, 70, 1000])
+    for (const view of [400, 720, 1100, 2100]) expect(railHeight(count, view, view)).toBe(240);
+  // Nor the prompts on screen: a view showing 50 short prompts at once keeps 240, and its list
+  // scrolls to keep them in view (railScrollTop).
+  for (const band of [1, 24, 50, 1000]) expect(railHeight(1000, 1000, 1000, 1000, band)).toBe(240);
+});
+
 test("wheel browsing belongs to the rail and is not snapped back by transcript scrolling", async () => {
   const view = await mount(70);
   try {
@@ -452,7 +459,7 @@ test("a wheel over a rail that scrolls itself is not the reader's: the transcrip
     input.mockClear();
     await act(async () => {
       list.dispatchEvent(new dom.window.WheelEvent("wheel", { deltaY: -120, bubbles: true, cancelable: true }));
-      tick(40).dispatchEvent(new dom.window.WheelEvent("wheel", { deltaY: -40, bubbles: true, cancelable: true }));
+      tick(10).dispatchEvent(new dom.window.WheelEvent("wheel", { deltaY: -40, bubbles: true, cancelable: true }));
     });
     expect(captured).toEqual([-120, -40]);
     expect(input).not.toHaveBeenCalled();

webviews/test/thread-minimap-wave-model.test.ts

@@ -97,6 +97,25 @@ describe("hover wave model", () => {
       expect(zipSettled(motions, 1000 + ZIP_MS)).toBe(true);
     });
 
+    test("a sweep keeps the bump whole: the rows ahead of the pointer grow as fast as it arrives", () => {
+      // Leo, 2026-11-04: "the missing chunks of the wave bother me". A sweep down a row every 16 ms:
+      // each frame, the hovered row's neighbours on both sides are already most of their way up, so no
+      // hole opens between the hovered tick and the rows it moves onto.
+      const motions = new Map<number, ZipMotion>();
+      const frame = 1000 / 60;
+      let now = 0;
+      for (let row = 10; row <= 30; row += 1, now += frame) {
+        zipRetarget(motions, row, 60, now);
+        const values = zipStep(motions, row, now + frame);
+        // One frame in, the row ahead has at least half its length (0.7) and the next at least a third
+        // of its (0.4): over ZIP_MS they had 30% and 18%.
+        expect(values.get(row + 1)!).toBeGreaterThan(0.7 * 0.5);
+        expect(values.get(row + 2)!).toBeGreaterThan(0.4 * 0.33);
+        // Shrinking ticks behind the pointer keep the zipper's tail and never outgrow the hovered one.
+        for (const [index, value] of values) if (index !== row) expect(value).toBeLessThan(values.get(row)!);
+      }
+    });
+
     test("a tick already heading to its target keeps its own motion", () => {
       const motions = new Map<number, ZipMotion>();
       zipRetarget(motions, 10, 30, 0);
@@ -114,7 +133,8 @@ describe("hover wave model", () => {
       const motions = new Map<number, ZipMotion>();
       zipRetarget(motions, 10, 30, 0);
       expect(motions.get(10)!.ms).toBe(ZIP_HOVER_MS);
-      expect(motions.get(11)!.ms).toBe(ZIP_MS);
+      // Its neighbours grow over the same quick step; only shrinking ticks take ZIP_MS.
+      expect(motions.get(11)!.ms).toBe(ZIP_HOVER_MS);
       zipStep(motions, 10, 1000);
       zipRetarget(motions, undefined, 30, 1000);
       for (const motion of motions.values()) expect(motion).toMatchObject({ to: 0, ms: ZIP_RELEASE_MS });

webviews/test/thread-rail-full-height.test.tsx

@@ -157,13 +157,18 @@ test("held ticks sit centered in the view, never stacked at the box's foot, whil
     // The first frame: twelve prompts held, the summary says forty-five.
     await render({ promptCount: 45 });
     const open = rail();
-    // The box holds all forty-five ticks, the thirty-three not held yet as resting ticks, centered: no
-    // empty rows above the held ones.
+    // The box is the rail's one height (240 px, the Codex rail's size) over all forty-five ticks, the
+    // thirty-three not held yet as resting ticks, centered: no empty rows above the held ones.
     expect(open.lead).toBe(0);
-    expect(open.height).toBe(450);
-    expect(document.querySelectorAll(".acpmux-minimap__entry").length).toBe(45);
-    expect(document.querySelectorAll(".acpmux-minimap__gap").length).toBe(0);
-    expect(open.top).toBeLessThanOrEqual(centered(450));
+    expect(open.height).toBe(240);
+    // Every prompt has its row, drawn or (past the list's view) a gap's.
+    const rows = [...document.querySelectorAll<HTMLElement>(".acpmux-minimap__entry, .acpmux-minimap__gap")].reduce(
+      (sum, node) =>
+        sum + (node.classList.contains("acpmux-minimap__gap") ? Number.parseFloat(node.style.height) / 10 : 1),
+      0,
+    );
+    expect(rows).toBe(45);
+    expect(open.top).toBeLessThanOrEqual(centered(240));
     // The ticks' run is about the view's middle, nowhere near its foot.
     expect(Math.abs(middle(open) - VIEWPORT / 2)).toBeLessThanOrEqual(100);
     // The latest tick ends the box, clear of the bottom stack.
@@ -174,8 +179,8 @@ test("held ticks sit centered in the view, never stacked at the box's foot, whil
     const full = rail();
     expect(full).toEqual(open);
     expect(full.lead).toBe(0);
-    expect(full.height).toBe(450);
-    expect(full.top).toBeLessThanOrEqual(centered(450));
+    expect(full.height).toBe(240);
+    expect(full.top).toBeLessThanOrEqual(centered(240));
     expect(Math.abs(middle(full) - VIEWPORT / 2)).toBeLessThanOrEqual(100);
   } finally {
     await act(async () => root.unmount());
@@ -198,9 +203,9 @@ test("without a count the rail grows as prompts arrive, as before", async () =>
         />,
       ),
     );
-    // Twelve prompts: the rail's 320 px floor, centered.
-    expect(rail().height).toBe(320);
-    expect(rail().top).toBeLessThanOrEqual(centered(320));
+    // Twelve prompts: the rail's one height, centered.
+    expect(rail().height).toBe(240);
+    expect(rail().top).toBeLessThanOrEqual(centered(240));
   } finally {
     await act(async () => root.unmount());
   }