fix(tmux): use actual pane dimensions and configured min width for grid calculation

Agent area width now uses real mainPane.width instead of hardcoded 50%
ratio. Grid planning, split availability, and spawn target finding now
respect user's agent_pane_min_width config instead of hardcoded
MIN_PANE_WIDTH=52, enabling 2-column grid layouts on narrower terminals.

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

@@ -112,6 +112,21 @@ describe("canSplitPaneAnyDirection", () => {
     // then
     expect(result).toBe(false)
   })
+
+  it("#given custom minPaneWidth #when pane fits smaller width #then returns true", () => {
+    //#given - pane too small for default MIN_PANE_WIDTH(52) but fits custom 30
+    const customMin = 30
+    const customMinSplitW = 2 * customMin + 1
+    const pane = createPane(customMinSplitW, MIN_SPLIT_HEIGHT - 1)
+
+    //#when
+    const defaultResult = canSplitPaneAnyDirection(pane)
+    const customResult = canSplitPaneAnyDirection(pane, customMin)
+
+    //#then
+    expect(defaultResult).toBe(false)
+    expect(customResult).toBe(true)
+  })
 })
 
 describe("getBestSplitDirection", () => {
@@ -179,6 +194,21 @@ describe("getBestSplitDirection", () => {
     // then
     expect(result).toBe("-v")
   })
+
+  it("#given custom minPaneWidth #when pane width below default but above custom #then returns -h", () => {
+    //#given
+    const customMin = 30
+    const customMinSplitW = 2 * customMin + 1
+    const pane = createPane(customMinSplitW, MIN_SPLIT_HEIGHT - 1)
+
+    //#when
+    const defaultResult = getBestSplitDirection(pane)
+    const customResult = getBestSplitDirection(pane, customMin)
+
+    //#then
+    expect(defaultResult).toBe(null)
+    expect(customResult).toBe("-h")
+  })
 })
 
 describe("decideSpawnActions", () => {
@@ -362,6 +392,20 @@ describe("calculateCapacity", () => {
     //#then
     expect(customCapacity.cols).toBeGreaterThanOrEqual(defaultCapacity.cols)
   })
+
+	it("#given non-50 main pane width #when calculating capacity #then uses real agent area width", () => {
+		//#given
+		const windowWidth = 220
+		const windowHeight = 44
+		const mainPaneWidth = 132
+
+		//#when
+		const capacity = calculateCapacity(windowWidth, windowHeight, 52, mainPaneWidth)
+
+		//#then
+		expect(capacity.cols).toBe(1)
+		expect(capacity.total).toBe(3)
+	})
 })
 
 describe("decideSpawnActions with custom agentPaneWidth", () => {
@@ -416,4 +460,40 @@ describe("decideSpawnActions with custom agentPaneWidth", () => {
       expect(result.actions[0].splitDirection).toBe("-h")
     }
   })
+
+	it("#given wider main pane #when capacity needs two evictions #then replace is chosen", () => {
+		//#given
+		const config: CapacityConfig = { mainPaneMinWidth: 120, agentPaneWidth: 40 }
+		const state = createWindowState(220, 44, [
+			{ paneId: "%1", width: 43, height: 44, left: 133, top: 0 },
+			{ paneId: "%2", width: 43, height: 44, left: 177, top: 0 },
+			{ paneId: "%3", width: 43, height: 21, left: 133, top: 22 },
+			{ paneId: "%4", width: 43, height: 21, left: 177, top: 22 },
+			{ paneId: "%5", width: 43, height: 21, left: 133, top: 33 },
+		])
+		state.mainPane = {
+			paneId: "%0",
+			width: 132,
+			height: 44,
+			left: 0,
+			top: 0,
+			title: "main",
+			isActive: true,
+		}
+		const mappings: SessionMapping[] = [
+			{ sessionId: "old-1", paneId: "%1", createdAt: new Date("2024-01-01") },
+			{ sessionId: "old-2", paneId: "%2", createdAt: new Date("2024-01-02") },
+			{ sessionId: "old-3", paneId: "%3", createdAt: new Date("2024-01-03") },
+			{ sessionId: "old-4", paneId: "%4", createdAt: new Date("2024-01-04") },
+			{ sessionId: "old-5", paneId: "%5", createdAt: new Date("2024-01-05") },
+		]
+
+		//#when
+		const result = decideSpawnActions(state, "ses-new", "new task", config, mappings)
+
+		//#then
+		expect(result.canSpawn).toBe(true)
+		expect(result.actions).toHaveLength(1)
+		expect(result.actions[0].type).toBe("replace")
+	})
 })

src/features/tmux-subagent/grid-planning.ts

@@ -28,8 +28,12 @@ export function calculateCapacity(
 	windowWidth: number,
 	windowHeight: number,
 	minPaneWidth: number = MIN_PANE_WIDTH,
+	mainPaneWidth?: number,
 ): GridCapacity {
-	const availableWidth = Math.floor(windowWidth * (1 - MAIN_PANE_RATIO))
+	const availableWidth =
+	typeof mainPaneWidth === "number"
+		? Math.max(0, windowWidth - mainPaneWidth - DIVIDER_SIZE)
+		: Math.floor(windowWidth * (1 - MAIN_PANE_RATIO))
 	const cols = Math.min(
 		MAX_GRID_SIZE,
 		Math.max(
@@ -55,8 +59,15 @@ export function computeGridPlan(
 	windowWidth: number,
 	windowHeight: number,
 	paneCount: number,
+	mainPaneWidth?: number,
+	minPaneWidth?: number,
 ): GridPlan {
-	const capacity = calculateCapacity(windowWidth, windowHeight)
+	const capacity = calculateCapacity(
+		windowWidth,
+		windowHeight,
+		minPaneWidth ?? MIN_PANE_WIDTH,
+		mainPaneWidth,
+	)
 	const { cols: maxCols, rows: maxRows } = capacity
 
 	if (maxCols === 0 || maxRows === 0 || paneCount === 0) {
@@ -79,7 +90,10 @@ export function computeGridPlan(
 		}
 	}
 
-	const availableWidth = Math.floor(windowWidth * (1 - MAIN_PANE_RATIO))
+	const availableWidth =
+	typeof mainPaneWidth === "number"
+		? Math.max(0, windowWidth - mainPaneWidth - DIVIDER_SIZE)
+		: Math.floor(windowWidth * (1 - MAIN_PANE_RATIO))
 	const slotWidth = Math.floor(availableWidth / bestCols)
 	const slotHeight = Math.floor(windowHeight / bestRows)
 

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

@@ -56,8 +56,8 @@ export function canSplitPane(
 	return pane.height >= MIN_SPLIT_HEIGHT
 }
 
-export function canSplitPaneAnyDirection(pane: TmuxPaneInfo): boolean {
-	return canSplitPaneAnyDirectionWithMinWidth(pane, MIN_PANE_WIDTH)
+export function canSplitPaneAnyDirection(pane: TmuxPaneInfo, minPaneWidth: number = MIN_PANE_WIDTH): boolean {
+	return canSplitPaneAnyDirectionWithMinWidth(pane, minPaneWidth)
 }
 
 export function canSplitPaneAnyDirectionWithMinWidth(

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

@@ -5,7 +5,7 @@ import type {
 	TmuxPaneInfo,
 	WindowState,
 } from "./types"
-import { MAIN_PANE_RATIO } from "./tmux-grid-constants"
+import { DIVIDER_SIZE } from "./tmux-grid-constants"
 import {
 	canSplitPane,
 	findMinimalEvictions,
@@ -26,7 +26,10 @@ export function decideSpawnActions(
 	}
 
 	const minPaneWidth = config.agentPaneWidth
-	const agentAreaWidth = Math.floor(state.windowWidth * (1 - MAIN_PANE_RATIO))
+	const agentAreaWidth = Math.max(
+		0,
+		state.windowWidth - state.mainPane.width - DIVIDER_SIZE,
+	)
 	const currentCount = state.agentPanes.length
 
 	if (agentAreaWidth < minPaneWidth) {

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

@@ -1,5 +1,4 @@
 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"
@@ -52,8 +51,14 @@ function findSplittableTarget(
 		return null
 	}
 
-	const plan = computeGridPlan(state.windowWidth, state.windowHeight, existingCount + 1)
-	const mainPaneWidth = Math.floor(state.windowWidth * MAIN_PANE_RATIO)
+	const plan = computeGridPlan(
+		state.windowWidth,
+		state.windowHeight,
+		existingCount + 1,
+		state.mainPane.width,
+		minPaneWidth,
+	)
+	const mainPaneWidth = state.mainPane.width
 	const occupancy = buildOccupancy(state.agentPanes, plan, mainPaneWidth)
 	const targetSlot = findFirstEmptySlot(occupancy, plan)