Syncing Modifier Keys for MouseWheel in ScottPlot WPF Control

Syncing Modifier Keys for MouseWheel in ScottPlot WPF Control

ScottPlot manages its own internal keyboard state via KeyDown/KeyUp events. Since these events require keyboard focus, modifier-dependent MouseWheel interactions (e.g., Ctrl+Wheel zoom) fail until the user clicks the control.

1. Problem Scenario

Symptoms

• Ctrl+MouseWheel zoom only works after clicking the control once
• MouseWheel events fire (mouse position-based), but modifier key state is not tracked
• Losing keyboard focus (clicking elsewhere) breaks Ctrl+Wheel again

Root Cause

WPF routes mouse events and keyboard events differently:

Event	Routing Basis	Requires Keyboard Focus
MouseWheel	Mouse position (hit-test)	No
MouseMove, MouseDown	Mouse position (hit-test)	No
KeyDown, KeyUp	Keyboard focus	Yes

ScottPlot tracks modifier keys through KeyDown/KeyUp into its internal KeyboardState. Without keyboard focus, KeyDown never fires, so the internal state never records Ctrl as pressed. The MouseWheel event arrives, but the zoom handler checks keys.IsPressed(Control) against the empty internal state and does nothing.

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2. Solution: Sync from Keyboard.Modifiers

System.Windows.Input.Keyboard.Modifiers reads the OS-level key state globally, regardless of which element has keyboard focus.

Inject the current modifier state into the control's input processor before processing each MouseWheel event:

using System.Windows;
using System.Windows.Input;

public static void ProcessMouseWheel(
    this UserInputProcessor processor,
    FrameworkElement fe,
    MouseWheelEventArgs e)
{
    // Sync modifier keys from WPF global state before processing
    SyncModifierKeys(processor);

    Pixel pixel = e.ToPixel(fe);

    IUserAction action = e.Delta > 0
        ? new MouseWheelUp(pixel)
        : new MouseWheelDown(pixel);

    processor.Process(action);
}

private static void SyncModifierKeys(UserInputProcessor processor)
{
    IUserAction action = (Keyboard.Modifiers & ModifierKeys.Control) != 0
        ? new KeyDown(StandardKeys.Control)
        : new KeyUp(StandardKeys.Control);

    processor.Process(action);
}

Key Points

• Keyboard.Modifiers is focus-independent (OS-level global query)
• Send KeyDown/KeyUp to the input processor to update its internal keyboard state
• Sync only the modifier keys actually used by the control's interaction responses
• Place the sync call before the MouseWheel action processing

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3. Why Not Focus on MouseEnter?

An alternative approach is to call Keyboard.Focus(this) on MouseEnter:

// Alternative approach - has side effects
protected override void OnMouseEnter(MouseEventArgs e)
{
    Keyboard.Focus(this);
    base.OnMouseEnter(e);
}

Approach	Pros	Cons
Sync Keyboard.Modifiers	No side effects, no focus stealing	Must be called on each wheel event
Focus on MouseEnter	Simple	Steals focus from TextBox/other inputs, disrupts Tab navigation

Prefer Keyboard.Modifiers sync - it solves the problem without affecting focus behavior.

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4. Common Mistakes

Only syncing on first use

// Wrong: sync once and cache
private static bool _synced = false;

private static void SyncModifierKeys(UserInputProcessor processor)
{
    if (_synced) return;  // Ctrl state can change between wheel events!
    _synced = true;
    // ...
}

Modifier state must be synced every time before processing MouseWheel, because the user may press or release Ctrl between wheel events.

Syncing after the wheel action

// Wrong: sync after processing
processor.Process(action);
SyncModifierKeys(processor);  // Too late - wheel already processed with stale state

The sync must happen before processor.Process(wheelAction).

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5. Checklist

• Identify ScottPlot's internal keyboard state mechanism (UserInputProcessor)
• Add Keyboard.Modifiers sync in the MouseWheel processing path
• Sync before the wheel action, not after
• Sync only the modifier keys used by ScottPlot's interaction responses
• Verify Ctrl+Wheel zoom works without clicking the ScottPlot control first
• Verify Ctrl+Wheel zoom still works after clicking elsewhere and returning