renderer: fix resize cleanup during pending footer transition

Author: simonklee

packages/core/src/renderer.ts

@@ -2696,14 +2696,16 @@ export class CliRenderer extends EventEmitter implements RenderContext {
       this.flushPendingSplitOutputBeforeTransition()
     }
 
+    const pendingSplitFooterTransition = this.pendingSplitFooterTransition
     const previousGeometry = calculateRenderGeometry(
       this._screenMode,
       this._terminalWidth,
       this._terminalHeight,
       this._footerHeight,
     )
     const prevWidth = this._terminalWidth
-    const visiblePreviousSplitHeight = this.pendingSplitFooterTransition?.sourceHeight ?? previousGeometry.effectiveFooterHeight
+    const previousTerminalHeight = this._terminalHeight
+    const visiblePreviousSplitHeight = pendingSplitFooterTransition?.sourceHeight ?? previousGeometry.effectiveFooterHeight
 
     this._terminalWidth = width
     this._terminalHeight = height
@@ -2721,9 +2723,23 @@ export class CliRenderer extends EventEmitter implements RenderContext {
     const splitFooterActive = this._screenMode === "split-footer"
 
     if (splitFooterActive) {
+      // Width shrink historically needs a broader scrub band, but if resize interrupts
+      // a deferred footer transition we also need to clear from that visible source surface.
+      let clearStart: number | null = null
+
       if (width < prevWidth && visiblePreviousSplitHeight > 0) {
-        const start = Math.max(this._terminalHeight - visiblePreviousSplitHeight * 2, 1)
-        const flush = ANSI.moveCursorAndClear(start, 1)
+        clearStart = Math.max(previousTerminalHeight - visiblePreviousSplitHeight * 2, 1)
+      }
+
+      if (pendingSplitFooterTransition !== null) {
+        clearStart =
+          clearStart === null
+            ? pendingSplitFooterTransition.sourceTopLine
+            : Math.min(clearStart, pendingSplitFooterTransition.sourceTopLine)
+      }
+
+      if (clearStart !== null) {
+        const flush = ANSI.moveCursorAndClear(clearStart, 1)
         this.writeOut(flush)
       }
 

packages/core/src/tests/renderer.console-startup.test.ts

@@ -1134,6 +1134,64 @@ test("CliRenderer split-footer resize cleanup uses the visible footer surface wh
   writeOutSpy.mockRestore()
 })
 
+test("CliRenderer split-footer resize cleanup uses the visible source top line across width and height resize while a deferred footer transition is pending", async () => {
+  const result = await createTestRenderer({
+    width: 40,
+    height: 10,
+    screenMode: "split-footer",
+    footerHeight: 4,
+    externalOutputMode: "capture-stdout",
+    consoleMode: "disabled",
+  })
+
+  renderer = result.renderer
+  ;(renderer as any)._terminalIsSetup = true
+
+  const writeOutSpy = spyOn(renderer as any, "writeOut")
+
+  renderer.footerHeight = 3
+
+  expect((renderer as any).pendingSplitFooterTransition).toEqual({
+    mode: "clear-stale-rows",
+    sourceTopLine: 2,
+    sourceHeight: 4,
+    targetTopLine: 2,
+    targetHeight: 3,
+  })
+
+  result.resize(20, 12)
+
+  expect(writeOutSpy).toHaveBeenCalledTimes(1)
+  expect(writeOutSpy.mock.calls[0]?.[0]).toBe(ANSI.moveCursorAndClear(2, 1))
+
+  writeOutSpy.mockRestore()
+})
+
+test("CliRenderer split-footer resize cleanup still clears the visible source surface when only height changes while a deferred footer transition is pending", async () => {
+  const result = await createTestRenderer({
+    width: 40,
+    height: 10,
+    screenMode: "split-footer",
+    footerHeight: 4,
+    externalOutputMode: "capture-stdout",
+    consoleMode: "disabled",
+  })
+
+  renderer = result.renderer
+  ;(renderer as any)._terminalIsSetup = true
+
+  const writeOutSpy = spyOn(renderer as any, "writeOut")
+
+  renderer.footerHeight = 3
+
+  result.resize(40, 12)
+
+  expect(writeOutSpy).toHaveBeenCalledTimes(1)
+  expect(writeOutSpy.mock.calls[0]?.[0]).toBe(ANSI.moveCursorAndClear(2, 1))
+
+  writeOutSpy.mockRestore()
+})
+
 test("CliRenderer split-footer footerHeight changes do not queue deferred transitions while startup cursor seeding blocks the first frame", async () => {
   const result = await createTestRenderer({
     width: 40,