Keep the main window floor on the animating setFrame path

Sources/App/CmuxMainWindow.swift

@@ -163,24 +163,34 @@ final class CmuxMainWindow: NSWindow {
         if inLiveResize {
             recordUserPlacement()
         }
-        guard !styleMask.contains(.fullScreen) else {
-            super.setFrame(frameRect, display: flag)
-            return
-        }
+        super.setFrame(frameWithinSizePolicy(frameRect), display: flag)
+    }
+
+    /// The animating variant is a separate entry point that does not route
+    /// through `setFrame(_:display:)`, so without this override display
+    /// placement, UI-test placement and automation could size the window
+    /// below the layout floor. Content that cannot shrink with it then
+    /// overflows the window, and a pane laid out past the window edge ends
+    /// at 0x0.
+    override func setFrame(_ frameRect: NSRect, display displayFlag: Bool, animate animateFlag: Bool) {
+        super.setFrame(frameWithinSizePolicy(frameRect), display: displayFlag, animate: animateFlag)
+    }
+
+    /// Caps a proposed frame to the display union and raises it to the
+    /// minimum content size. Full screen frames belong to the system.
+    private func frameWithinSizePolicy(_ frameRect: NSRect) -> NSRect {
+        guard !styleMask.contains(.fullScreen) else { return frameRect }
         let capped = Self.frameByCappingOversizedDimensions(
             frameRect,
             displayFrames: NSScreen.screens.map {
                 (frame: $0.frame, visibleFrame: $0.visibleFrame)
             }
         )
-        super.setFrame(
-            Self.frameByRaisingUndersizedDimensions(
-                capped,
-                minimumSize: Self.minimumContentSize,
-                currentFrame: frame,
-                isLiveResize: inLiveResize
-            ),
-            display: flag
+        return Self.frameByRaisingUndersizedDimensions(
+            capped,
+            minimumSize: Self.minimumContentSize,
+            currentFrame: frame,
+            isLiveResize: inLiveResize
         )
     }
 

cmuxTests/MainWindowSelfSizingTests.swift

@@ -227,6 +227,45 @@ final class MainWindowSelfSizingTests: XCTestCase {
         )
     }
 
+    /// `setFrame(_:display:animate:)` is its own entry point: it does not
+    /// route through `setFrame(_:display:)`, so display placement, UI-test
+    /// placement and automation that use it once shrank the window below
+    /// the floor. The content could not follow, overflowed the window, and a
+    /// split pane laid out past its bottom edge ended at 0x0 (#15347).
+    @MainActor
+    func testAnimatingSetFrameRaisesUndersizedFrameToMinimumContentSize() {
+        for animate in [false, true] {
+            let window = CmuxMainWindow(
+                contentRect: NSRect(x: 0, y: 0, width: 900, height: 600),
+                styleMask: [.titled, .closable, .miniaturizable, .resizable, .fullSizeContentView],
+                backing: .buffered,
+                defer: false
+            )
+            window.isReleasedWhenClosed = false
+            defer {
+                window.orderOut(nil)
+                window.close()
+            }
+
+            let minimum = CmuxMainWindow.minimumContentSize
+            let undersized = NSRect(x: 80, y: 500, width: 800, height: 200)
+            window.setFrame(undersized, display: false, animate: animate)
+
+            XCTAssertEqual(
+                window.frame.width, 800, accuracy: 0.5,
+                "animate=\(animate): a width above the floor must be kept"
+            )
+            XCTAssertGreaterThanOrEqual(
+                window.frame.height, minimum.height - 0.5,
+                "animate=\(animate): a setFrame below the minimum height must be raised to the floor"
+            )
+            XCTAssertEqual(
+                window.frame.maxY, undersized.maxY, accuracy: 0.5,
+                "animate=\(animate): raising an undersized frame must keep the top edge and extend downward"
+            )
+        }
+    }
+
     /// Ordinary frames must flow through the undersized raise untouched so
     /// user-owned placement (partial off-screen, multi-display) is never
     /// perturbed by the floor.

dogfood/fuzz/regressions/issue-15347-stacked-panes-short-window.json

@@ -0,0 +1,27 @@
+{
+ "kind": "cmux-fuzz-repro",
+ "version": 1,
+ "signature": {
+  "kind": "invariant",
+  "key": "pane-view-degenerate",
+  "title": "Layout invariant broken: pane-view-degenerate",
+  "digest": "3ad440fc1652"
+ },
+ "steps": [
+  {
+   "do": "split",
+   "dir": "d",
+   "via": "socket"
+  },
+  {
+   "do": "split",
+   "dir": "u",
+   "via": "socket"
+  },
+  {
+   "do": "window_resize",
+   "w": 800,
+   "h": 200
+  }
+ ]
+}