How to set custom height for keyboard extension without resize flicker?

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Description

I'm developing a custom keyboard extension using UIInputViewController and need to set a specific height of 268 points. The keyboard functions correctly, but there's a visible flicker and resize animation during launch that I cannot eliminate.

The Problem

When the keyboard launches, iOS provides incorrect heights before settling on the correct one. At launch, the view starts at 0×0. Around 295ms later, iOS sets the frame to 440×956 which is full screen height and wrong. Around 373ms, iOS changes it to 440×452 which is still wrong. Finally around 390ms, iOS settles at 440×268 which matches our constraint.

This causes visible flicker as the view resizes three times rapidly. The keyboard appears to shrink from full screen down to the correct height, and users can clearly see this animation happening.

What I've Tried

I've tried adding a height constraint on self.view which gives me the correct height but causes the visible flicker.

I created a custom UIInputView subclass and overrode intrinsicContentSize to return my desired height. iOS completely ignores this and gives random heights like 471pt, 680pt, or 956pt instead.

I set allowsSelfSizing to true on my UIInputView subclass. iOS ignores this property.

I set preferredContentSize on the view controller. iOS ignores this as well.

I tried adding the constraint in viewDidAppear instead of viewDidLoad, thinking iOS might have settled by then. It still causes flicker.

I overrode the frame and bounds setters on my UIInputView to clamp the height to my desired value. iOS bypasses these overrides somehow.

I overrode layoutSubviews to force the correct height after the super call. iOS still applies its own height.

Specific Question

What is the correct API or technique to specify a keyboard extension's height that iOS will respect immediately upon launch, without triggering the resize animation sequence?

Other third-party keyboards like Grammarly and SwiftKey appear to have solved this problem. Their keyboards appear at the correct height without any visible flicker. How do they achieve this?

Expected Outcome

The keyboard should appear at 268pt height on the first frame with no visible resize animation.

Steps to Reproduce

Create a new iOS App project in Xcode and add a Keyboard Extension target. In KeyboardViewController.swift, add a height constraint in viewDidLoad:

override func viewDidLoad() {
    super.viewDidLoad()

    let heightConstraint = view.heightAnchor.constraint(equalToConstant: 268)
    heightConstraint.priority = .defaultHigh
    heightConstraint.isActive = true

    let label = UILabel()
    label.text = "Demo Keyboard"
    label.textAlignment = .center
    label.translatesAutoresizingMaskIntoConstraints = false
    view.addSubview(label)
    NSLayoutConstraint.activate([
        label.centerXAnchor.constraint(equalTo: view.centerXAnchor),
        label.centerYAnchor.constraint(equalTo: view.centerYAnchor)
    ])
}

Build and run on a physical device. Enable the keyboard in Settings, then General, then Keyboard, then Keyboards. Open any app with a text field and switch to the custom keyboard using the globe button. Observe the height changing from around 956pt to 452pt to 268pt with visible animation.

Environment

iOS 17 and iOS 18 and 26.2, Xcode 16 and Xcode 26, affects all iPhone models tested, reproducible on both simulator and physical device.

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