Fix IME firstRect preedit anchor sizing (https://github.com/manaflow-ai/cmux/issues/265)

Sources/GhosttyTerminalView.swift

@@ -2115,6 +2115,17 @@ class GhosttyNSView: NSView, NSUserInterfaceValidations {
     var keyTextAccumulatorForTesting: [String]? {
         keyTextAccumulator
     }
+
+    // Test-only IME point override so firstRect behavior can be regression tested.
+    private var imePointOverrideForTesting: (x: Double, y: Double, width: Double, height: Double)?
+
+    func setIMEPointForTesting(x: Double, y: Double, width: Double, height: Double) {
+        imePointOverrideForTesting = (x, y, width, height)
+    }
+
+    func clearIMEPointForTesting() {
+        imePointOverrideForTesting = nil
+    }
 #endif
 
 #if DEBUG
@@ -4204,17 +4215,28 @@ extension GhosttyNSView: NSTextInputClient {
         // Use Ghostty's IME point API for accurate cursor position if available.
         var x: Double = 0
         var y: Double = 0
-        var w: Double = 0
-        var h: Double = 0
+        var w: Double = cellSize.width
+        var h: Double = cellSize.height
+#if DEBUG
+        if let override = imePointOverrideForTesting {
+            x = override.x
+            y = override.y
+            w = override.width
+            h = override.height
+        } else if let surface = surface {
+            ghostty_surface_ime_point(surface, &x, &y, &w, &h)
+        }
+#else
         if let surface = surface {
             ghostty_surface_ime_point(surface, &x, &y, &w, &h)
         }
+#endif
 
         // Ghostty coordinates are top-left origin; AppKit expects bottom-left.
         let viewRect = NSRect(
             x: x,
             y: frame.size.height - y,
-            width: 0,
+            width: w,
             height: max(h, cellSize.height)
         )
         let winRect = convert(viewRect, to: nil)

cmuxTests/CJKIMEInputTests.swift

@@ -642,6 +642,73 @@ final class CJKIMECompositionSequenceTests: XCTestCase {
     }
 }
 
+// MARK: - IME firstRect placement and sizing
+
+/// Regression tests for IME candidate/preedit anchor rectangle reporting.
+/// If width/height are discarded here, macOS can place preedit UI incorrectly.
+final class CJKIMEFirstRectTests: XCTestCase {
+
+    func testFirstRectUsesIMEProvidedWidthAndHeight() {
+        let frame = NSRect(x: 0, y: 0, width: 800, height: 600)
+        let view = GhosttyNSView(frame: frame)
+        view.cellSize = CGSize(width: 10, height: 20)
+        view.setIMEPointForTesting(x: 120, y: 240, width: 64, height: 26)
+
+        let window = NSWindow(
+            contentRect: NSRect(x: 100, y: 100, width: 800, height: 600),
+            styleMask: [.titled],
+            backing: .buffered,
+            defer: false
+        )
+        let content = NSView(frame: frame)
+        window.contentView = content
+        content.addSubview(view)
+        view.frame = frame
+
+        defer {
+            view.clearIMEPointForTesting()
+            window.orderOut(nil)
+        }
+
+        let rect = view.firstRect(forCharacterRange: NSRange(location: 0, length: 1), actualRange: nil)
+
+        let expectedViewRect = NSRect(x: 120, y: frame.height - 240, width: 64, height: 26)
+        let expectedScreenRect = window.convertToScreen(view.convert(expectedViewRect, to: nil))
+
+        XCTAssertEqual(rect.origin.x, expectedScreenRect.origin.x, accuracy: 0.001)
+        XCTAssertEqual(rect.origin.y, expectedScreenRect.origin.y, accuracy: 0.001)
+        XCTAssertEqual(rect.width, 64, accuracy: 0.001)
+        XCTAssertEqual(rect.height, 26, accuracy: 0.001)
+    }
+
+    func testFirstRectFallsBackToCellHeightWhenIMEHeightIsZero() {
+        let frame = NSRect(x: 0, y: 0, width: 640, height: 480)
+        let view = GhosttyNSView(frame: frame)
+        view.cellSize = CGSize(width: 9, height: 18)
+        view.setIMEPointForTesting(x: 80, y: 120, width: 36, height: 0)
+
+        let window = NSWindow(
+            contentRect: NSRect(x: 40, y: 40, width: 640, height: 480),
+            styleMask: [.titled],
+            backing: .buffered,
+            defer: false
+        )
+        let content = NSView(frame: frame)
+        window.contentView = content
+        content.addSubview(view)
+        view.frame = frame
+
+        defer {
+            view.clearIMEPointForTesting()
+            window.orderOut(nil)
+        }
+
+        let rect = view.firstRect(forCharacterRange: NSRange(location: 0, length: 1), actualRange: nil)
+        XCTAssertEqual(rect.width, 36, accuracy: 0.001)
+        XCTAssertEqual(rect.height, 18, accuracy: 0.001)
+    }
+}
+
 // MARK: - Key text accumulator during CJK IME composition
 
 /// Tests that the keyTextAccumulator correctly manages text during the keyDown