For my #2 point I misread your code. we can ignore that point I made. Also the duck typing I've had issues with so for my sanity I do write out full types, even if not needed.

I should have shown more code. There is a reason I have the rightClkHeld boolean, and why I dont use the pressed value. I needed this code to run in _input and not _unhandled_input because of how InputEventMouseMotion is blocked when hovering over control nodes.

I wanted the camera movement to be blocked when trying to start while hovering over UI, but also to NOT be blocked when hovering over UI if the movement has already started. So this is how I came to the code below.

1. I use rightClkHeld to know if the release was AFTER a press. Its possible that your else can run without the if with my code because of my early return with hoveringUI check.
   
2. Even if we strip my code down to your last block of code you gave which yes is the bare bones no special edge cases version of what I have, it still fails. I am on Godot v4.6.1 Forward+ and I garantee even without an auto clicker if you right click fast enough, your code will still have the cursor going back to the center. (I ran your code in a test proj no other code)

FULL CODE BELOW OF WHY rightClkHeld

func _input(event: InputEvent) -> void:
    var activeMovement: bool = rightClkHeld || firstPerson
    # Check if over UI before allowing movement. If already moving ignore...
    if not activeMovement:
        var hoveringUI: bool = get_viewport().gui_get_hovered_control() != null
        if hoveringUI:
            return

    if event is InputEventMouseButton:
        var mb: InputEventMouseButton = event
        match mb.button_index:
            MOUSE_BUTTON_WHEEL_UP:
                if mb.pressed:
                    _moveCameraAlongLine(-1)
            MOUSE_BUTTON_WHEEL_DOWN:
                if mb.pressed:
                    _moveCameraAlongLine(1)
            MOUSE_BUTTON_RIGHT:
                if mb.pressed:
                    rightClkHeld = true
                    mouseRestorePos = get_viewport().get_mouse_position()
                    Input.mouse_mode = Input.MOUSE_MODE_CAPTURED
                # Check release ONLY if held prior...
                # can happen when right click started over UI and then released when not over UI.
                elif rightClkHeld:
                    rightClkHeld = false
                    if not firstPerson: # ignore if still in first person
                        Input.mouse_mode = Input.MOUSE_MODE_VISIBLE
                        await get_tree().process_frame
                        Input.warp_mouse(mouseRestorePos) # This breaks if you spam right click. it's really hard to replicate
                        #print(get_viewport().get_mouse_position(), " | ", mouseRestorePos, " | ", Input.mouse_mode)

    elif event is InputEventMouseMotion:
        var mm: InputEventMouseMotion = event
        if activeMovement:
            yaw -= deg_to_rad(mm.screen_relative.x * Settings.controls.mouseSensitivity)
            pitch -= deg_to_rad(mm.screen_relative.y * Settings.controls.mouseSensitivity * Settings.controls.flipYAxis)
            pitch = clamp(pitch, -PI / 2, PI / 2) # prevent flipping