@@ -29,6 +29,7 @@ import {
   parseLastCredentialTag,
   parseLastQuestionTag,
 } from '@/app/workspace/[workspaceId]/home/components/message-content/components/special-tags'
+import { nextSizerFloor } from '@/app/workspace/[workspaceId]/home/components/mothership-chat/sizer-floor'
 import { QueuedMessages } from '@/app/workspace/[workspaceId]/home/components/queued-messages'
 import {
   UserInput,
@@ -328,6 +329,8 @@ export function MothershipChat({
   const sizerRef = useRef<HTMLDivElement | null>(null)
   const scrollerPaddingRef = useRef<{ top: number; bottom: number } | null>(null)
   const sizerFloorAppliedRef = useRef(0)
+  const heldHighWaterRef = useRef(0)
+  const floorChatRef = useRef<string | undefined>(undefined)
   const floorDrainRafRef = useRef(0)
   useEffect(() => () => cancelAnimationFrame(floorDrainRafRef.current), [])
 
@@ -337,11 +340,11 @@ export function MothershipChat({
    * row-height shrinks; when they pull scrollHeight under
    * `scrollTop + clientHeight`, the browser clamps `scrollTop` and the pinned
    * transcript visibly drops, then the chase glides it back. Flooring the
-   * sizer at exactly the scrolled-to extent prevents that clamp while never
-   * ADDING space — the floor cannot exceed what is already on screen. So an
-   * estimate correction (a fresh row measuring smaller than
-   * ROW_HEIGHT_ESTIMATE) releases immediately instead of holding phantom space
-   * the chase would scroll into and bounce back out of.
+   * sizer prevents that clamp while never ADDING space, so an estimate
+   * correction (a fresh row measuring smaller than ROW_HEIGHT_ESTIMATE)
+   * releases immediately instead of holding phantom space the chase would
+   * scroll into and bounce back out of. {@link nextSizerFloor} owns the value
+   * and the invariant that keeps it honest.
    *
    * Active on the same signal as auto-scroll: the reveal keeps re-parsing
    * markdown (and shrinking) after the network stream closes, so the floor
@@ -361,7 +364,26 @@ export function MothershipChat({
     const sizer = sizerRef.current
     const el = scrollElementRef.current
     if (!sizer || !el) return
+    // A chat switch replaces the entire transcript, so a floor held for the
+    // previous one is meaningless — and its high-water mark would otherwise
+    // hand a short chat the tall chat's space for as long as the outgoing
+    // turn's `lastRowAnimating` keeps the floor engaged. Released outright
+    // rather than drained: the switch re-lands the viewport anyway, so there
+    // is no eased settle to preserve. A pending chat adopting its id is the
+    // SAME conversation, so it must not release mid-turn.
+    if (floorChatRef.current !== chatId) {
+      const isPendingPersist = floorChatRef.current === undefined && chatId !== undefined
+      floorChatRef.current = chatId
+      if (!isPendingPersist) {
+        cancelAnimationFrame(floorDrainRafRef.current)
+        floorDrainRafRef.current = 0
+        sizerFloorAppliedRef.current = 0
+        heldHighWaterRef.current = 0
+        sizer.style.minHeight = ''
+      }
+    }
     if (!floorActive) {
+      heldHighWaterRef.current = 0
       if (sizerFloorAppliedRef.current === 0) return
       // A drain already in flight keeps its own rAF cadence — settle-burst
       // commits re-enter this branch and must not add extra steps in layout,
@@ -405,14 +427,16 @@ export function MothershipChat({
       }
     }
     const padding = scrollerPaddingRef.current
-    // Math.floor, not the raw float: a fractional min-height can round
-    // scrollHeight 1px ABOVE the scrolled-to extent, and that phantom 1px gap
-    // re-derives 1px higher after every chase step — a visible 1px/frame
-    // upward creep whenever the floor is what's holding scrollHeight.
-    const floor = Math.max(
-      0,
-      Math.floor(el.scrollTop + el.clientHeight - padding.top - padding.bottom)
-    )
+    const { floor, highWater } = nextSizerFloor({
+      previousHighWater: heldHighWaterRef.current,
+      appliedFloor: sizerFloorAppliedRef.current,
+      contentHeight: virtualizer.getTotalSize(),
+      scrollTop: el.scrollTop,
+      clientHeight: el.clientHeight,
+      paddingTop: padding.top,
+      paddingBottom: padding.bottom,
+    })
+    heldHighWaterRef.current = highWater
     // Dead-band: the floor feeds back into its own inputs (a floored value can
     // land a fraction BELOW the extent, the browser clamps scrollTop, and the
     // next commit re-derives from the clamped position — a visible ~1px×N
 
@@ -0,0 +1,90 @@
+/**
+ * @vitest-environment node
+ */
+import { describe, expect, it } from 'vitest'
+import { nextSizerFloor } from '@/app/workspace/[workspaceId]/home/components/mothership-chat/sizer-floor'
+
+/** Matches the transcript scroller's `pt-4 pb-2`. */
+const PADDING = { paddingTop: 16, paddingBottom: 8 }
+/** A viewport scrolled to the bottom of 2000px of content: extent resolves to 2000. */
+const PINNED = { scrollTop: 1424, clientHeight: 600, ...PADDING }
+
+describe('nextSizerFloor', () => {
+  it('follows the scrolled-to extent while it stays under the high-water mark', () => {
+    const { floor } = nextSizerFloor({
+      ...PINNED,
+      previousHighWater: 5000,
+      appliedFloor: 0,
+      contentHeight: 5000,
+    })
+    expect(floor).toBe(2000)
+  })
+
+  it('floors fractional scroll offsets down so the sizer cannot round above the extent', () => {
+    const { floor } = nextSizerFloor({
+      scrollTop: 100.7,
+      clientHeight: 600,
+      ...PADDING,
+      previousHighWater: 5000,
+      appliedFloor: 0,
+      contentHeight: 5000,
+    })
+    expect(floor).toBe(676)
+  })
+
+  it('never returns a negative floor for a container smaller than its padding', () => {
+    const { floor } = nextSizerFloor({
+      scrollTop: 0,
+      clientHeight: 8,
+      ...PADDING,
+      previousHighWater: 500,
+      appliedFloor: 0,
+      contentHeight: 500,
+    })
+    expect(floor).toBe(0)
+  })
+
+  it('never exceeds the content height when the transcript is shorter than the viewport', () => {
+    const { floor } = nextSizerFloor({
+      scrollTop: 0,
+      clientHeight: 600,
+      ...PADDING,
+      previousHighWater: 0,
+      appliedFloor: 0,
+      contentHeight: 180,
+    })
+    expect(floor).toBe(180)
+  })
+
+  it('holds the high-water mark when content re-measures smaller mid-turn', () => {
+    const { floor, highWater } = nextSizerFloor({
+      ...PINNED,
+      previousHighWater: 2000,
+      appliedFloor: 2000,
+      contentHeight: 1940,
+    })
+    expect(highWater).toBe(2000)
+    expect(floor).toBe(2000)
+  })
+
+  it('carries undrained debt across a turn boundary that interrupts the drain', () => {
+    const { floor, highWater } = nextSizerFloor({
+      ...PINNED,
+      previousHighWater: 0,
+      appliedFloor: 1985,
+      contentHeight: 1940,
+    })
+    expect(highWater).toBe(1985)
+    expect(floor).toBe(1985)
+  })
+
+  it('raises the high-water mark as content grows', () => {
+    const { highWater } = nextSizerFloor({
+      ...PINNED,
+      previousHighWater: 1200,
+      appliedFloor: 1200,
+      contentHeight: 1600,
+    })
+    expect(highWater).toBe(1600)
+  })
+})
@@ -0,0 +1,64 @@
+interface SizerFloorInput {
+  /** High-water mark carried from the previous commit of this turn. */
+  previousHighWater: number
+  /** Floor currently written to the sizer, including one a drain has not finished releasing. */
+  appliedFloor: number
+  /** Natural sizer height — `virtualizer.getTotalSize()`. */
+  contentHeight: number
+  scrollTop: number
+  clientHeight: number
+  paddingTop: number
+  paddingBottom: number
+}
+
+interface SizerFloorResult {
+  /** Floor to write to the sizer's `min-height`. */
+  floor: number
+  /** High-water mark to carry into the next commit. */
+  highWater: number
+}
+
+/**
+ * Floor height for the transcript sizer while a turn streams — the value that
+ * keeps `scrollHeight` from dipping below the scrolled-to extent when a row
+ * transiently re-measures smaller — together with the high-water mark that
+ * bounds it.
+ *
+ * The extent (`scrollTop + clientHeight`) is the viewport's bottom edge, which
+ * sits BELOW the content whenever the transcript is shorter than the viewport —
+ * early in a turn, or in any short chat. Flooring at the raw extent invents
+ * scrollable space no content occupies, which stays invisible only while the
+ * container keeps its height. The moment it shrinks mid-turn — the composer
+ * growing, the queued-message banner appearing, a window or panel resize — that
+ * space becomes real scrollable room, the bottom-pin scrolls into it, and the
+ * transcript is dragged upward until the floor drains at the end of the turn.
+ *
+ * The high-water mark is what the extent is clamped to, and it folds in the
+ * APPLIED floor as well as the live content height. Both terms are load-bearing:
+ *
+ * - Live content alone would release the debt on the very commit that created
+ *   it, since holding space a shrink just took away is the floor's whole purpose.
+ * - Ignoring the applied floor would dump undrained debt in a single frame when
+ *   a queued message re-engages the floor mid-drain — the end-of-turn jump the
+ *   eased drain exists to prevent.
+ *
+ * Together they say: never exceed the space content has actually held this turn.
+ *
+ * `Math.floor`, not the raw float: a fractional min-height can round
+ * `scrollHeight` 1px ABOVE the scrolled-to extent, and that phantom 1px gap
+ * re-derives 1px higher after every chase step — a visible 1px/frame upward
+ * creep whenever the floor is what is holding `scrollHeight`.
+ */
+export function nextSizerFloor({
+  previousHighWater,
+  appliedFloor,
+  contentHeight,
+  scrollTop,
+  clientHeight,
+  paddingTop,
+  paddingBottom,
+}: SizerFloorInput): SizerFloorResult {
+  const highWater = Math.max(previousHighWater, contentHeight, appliedFloor)
+  const extent = Math.max(0, Math.floor(scrollTop + clientHeight - paddingTop - paddingBottom))
+  return { floor: Math.min(extent, highWater), highWater }
+}