fix: align split targeting with configured pane width

Use the configured agent pane width consistently in split target selection and avoid close+spawn churn by replacing the oldest pane when eviction is required.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/features/tmux-subagent/decision-engine.test.ts

@@ -228,7 +228,7 @@ describe("decideSpawnActions", () => {
       expect(result.actions[0].type).toBe("spawn")
     })
 
-    it("closes oldest pane when existing panes are too small to split", () => {
+    it("replaces oldest pane when existing panes are too small to split", () => {
       // given - existing pane is below minimum splittable size
       const state = createWindowState(220, 30, [
         { paneId: "%1", width: 50, height: 15, left: 110, top: 0 },
@@ -242,9 +242,8 @@ describe("decideSpawnActions", () => {
 
       // then
       expect(result.canSpawn).toBe(true)
-      expect(result.actions.length).toBe(2)
-      expect(result.actions[0].type).toBe("close")
-      expect(result.actions[1].type).toBe("spawn")
+      expect(result.actions.length).toBe(1)
+      expect(result.actions[0].type).toBe("replace")
     })
 
     it("can spawn when existing pane is large enough to split", () => {
@@ -394,4 +393,27 @@ describe("decideSpawnActions with custom agentPaneWidth", () => {
     expect(defaultResult.canSpawn).toBe(false)
     expect(customResult.canSpawn).toBe(true)
   })
+
+  it("#given custom agentPaneWidth and splittable existing pane #when deciding spawn #then uses spawn without eviction", () => {
+    //#given
+    const customConfig: CapacityConfig = { mainPaneMinWidth: 120, agentPaneWidth: 40 }
+    const state = createWindowState(220, 44, [
+      { paneId: "%1", width: 90, height: 30, left: 110, top: 0 },
+    ])
+    const mappings: SessionMapping[] = [
+      { sessionId: "old-ses", paneId: "%1", createdAt: new Date("2024-01-01") },
+    ]
+
+    //#when
+    const result = decideSpawnActions(state, "ses1", "test", customConfig, mappings)
+
+    //#then
+    expect(result.canSpawn).toBe(true)
+    expect(result.actions.length).toBe(1)
+    expect(result.actions[0].type).toBe("spawn")
+    if (result.actions[0].type === "spawn") {
+      expect(result.actions[0].targetPaneId).toBe("%1")
+      expect(result.actions[0].splitDirection).toBe("-h")
+    }
+  })
 })

src/features/tmux-subagent/pane-split-availability.ts

@@ -57,11 +57,21 @@ export function canSplitPane(
 }
 
 export function canSplitPaneAnyDirection(pane: TmuxPaneInfo): boolean {
-	return pane.width >= MIN_SPLIT_WIDTH || pane.height >= MIN_SPLIT_HEIGHT
+	return canSplitPaneAnyDirectionWithMinWidth(pane, MIN_PANE_WIDTH)
 }
 
-export function getBestSplitDirection(pane: TmuxPaneInfo): SplitDirection | null {
-	const canH = pane.width >= MIN_SPLIT_WIDTH
+export function canSplitPaneAnyDirectionWithMinWidth(
+	pane: TmuxPaneInfo,
+	minPaneWidth: number = MIN_PANE_WIDTH,
+): boolean {
+	return pane.width >= minSplitWidthFor(minPaneWidth) || pane.height >= MIN_SPLIT_HEIGHT
+}
+
+export function getBestSplitDirection(
+	pane: TmuxPaneInfo,
+	minPaneWidth: number = MIN_PANE_WIDTH,
+): SplitDirection | null {
+	const canH = pane.width >= minSplitWidthFor(minPaneWidth)
 	const canV = pane.height >= MIN_SPLIT_HEIGHT
 
 	if (!canH && !canV) return null

src/features/tmux-subagent/spawn-action-decider.ts

@@ -62,7 +62,7 @@ export function decideSpawnActions(
 	}
 
 	if (isSplittableAtCount(agentAreaWidth, currentCount, minPaneWidth)) {
-		const spawnTarget = findSpawnTarget(state)
+		const spawnTarget = findSpawnTarget(state, minPaneWidth)
 		if (spawnTarget) {
 			return {
 				canSpawn: true,
@@ -85,19 +85,14 @@ export function decideSpawnActions(
 			canSpawn: true,
 			actions: [
 				{
-					type: "close",
+					type: "replace",
 					paneId: oldestPane.paneId,
-					sessionId: oldestMapping?.sessionId || "",
-				},
-				{
-					type: "spawn",
-					sessionId,
+					oldSessionId: oldestMapping?.sessionId || "",
+					newSessionId: sessionId,
 					description,
-					targetPaneId: state.mainPane.paneId,
-					splitDirection: "-h",
 				},
 			],
-			reason: "closed 1 pane to make room for split",
+			reason: "replaced oldest pane to avoid split churn",
 		}
 	}
 

src/features/tmux-subagent/spawn-target-finder.ts

@@ -2,6 +2,7 @@ import type { SplitDirection, TmuxPaneInfo, WindowState } from "./types"
 import { MAIN_PANE_RATIO } from "./tmux-grid-constants"
 import { computeGridPlan, mapPaneToSlot } from "./grid-planning"
 import { canSplitPane, getBestSplitDirection } from "./pane-split-availability"
+import { MIN_PANE_WIDTH } from "./types"
 
 export interface SpawnTarget {
 	targetPaneId: string
@@ -37,6 +38,7 @@ function findFirstEmptySlot(
 
 function findSplittableTarget(
 	state: WindowState,
+	minPaneWidth: number,
 	_preferredDirection?: SplitDirection,
 ): SpawnTarget | null {
 	if (!state.mainPane) return null
@@ -44,7 +46,7 @@ function findSplittableTarget(
 
 	if (existingCount === 0) {
 		const virtualMainPane: TmuxPaneInfo = { ...state.mainPane, width: state.windowWidth }
-		if (canSplitPane(virtualMainPane, "-h")) {
+		if (canSplitPane(virtualMainPane, "-h", minPaneWidth)) {
 			return { targetPaneId: state.mainPane.paneId, splitDirection: "-h" }
 		}
 		return null
@@ -56,17 +58,17 @@ function findSplittableTarget(
 	const targetSlot = findFirstEmptySlot(occupancy, plan)
 
 	const leftPane = occupancy.get(`${targetSlot.row}:${targetSlot.col - 1}`)
-	if (leftPane && canSplitPane(leftPane, "-h")) {
+	if (leftPane && canSplitPane(leftPane, "-h", minPaneWidth)) {
 		return { targetPaneId: leftPane.paneId, splitDirection: "-h" }
 	}
 
 	const abovePane = occupancy.get(`${targetSlot.row - 1}:${targetSlot.col}`)
-	if (abovePane && canSplitPane(abovePane, "-v")) {
+	if (abovePane && canSplitPane(abovePane, "-v", minPaneWidth)) {
 		return { targetPaneId: abovePane.paneId, splitDirection: "-v" }
 	}
 
 	const splittablePanes = state.agentPanes
-		.map((pane) => ({ pane, direction: getBestSplitDirection(pane) }))
+		.map((pane) => ({ pane, direction: getBestSplitDirection(pane, minPaneWidth) }))
 		.filter(
 			(item): item is { pane: TmuxPaneInfo; direction: SplitDirection } =>
 				item.direction !== null,
@@ -81,6 +83,9 @@ function findSplittableTarget(
 	return null
 }
 
-export function findSpawnTarget(state: WindowState): SpawnTarget | null {
-	return findSplittableTarget(state)
+export function findSpawnTarget(
+	state: WindowState,
+	minPaneWidth: number = MIN_PANE_WIDTH,
+): SpawnTarget | null {
+	return findSplittableTarget(state, minPaneWidth)
 }