Why does FSceneViewport::OnMouseButtonDown() handle only the right mouse button in a special way?

Development Programming & Scripting C++
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1

I’m looking at the code for FSceneViewport::OnMouseButtonDown(). But I have a question about following code.

Why is FViewportClient::InputKey() supposed to run regardless of MouseCaptureMode only when the right mouse button is pressed?

const bool bTemporaryCapture = ViewportClient->GetMouseCaptureMode() == EMouseCaptureMode::CaptureDuringMouseDown || (ViewportClient->GetMouseCaptureMode() == EMouseCaptureMode::CaptureDuringRightMouseDown && InMouseEvent.GetEffectingButton() == EKeys::RightMouseButton);

// Process primary input if we aren't currently a game viewport, we already have capture, or we are permanent capture that doesn't consume the mouse down.
const bool bProcessInputPrimary = !IsCurrentlyGameViewport() || HasMouseCapture() || (ViewportClient->GetMouseCaptureMode() == EMouseCaptureMode::CapturePermanently_IncludingInitialMouseDown);

const bool bAnyMenuWasVisible = FSlateApplication::Get().AnyMenusVisible();

// Process the mouse event
if (bTemporaryCapture || bProcessInputPrimary)
{
	if (!ViewportClient->InputKey(FInputKeyEventArgs(this, InMouseEvent.GetUserIndex(), InMouseEvent.GetEffectingButton(), IE_Pressed, 1.0f, InMouseEvent.IsTouchEvent())))
	{
		CurrentReplyState = FReply::Unhandled();
	}
}

Additionally, I also looked at the code in FSceneViewport::OnMouseButtonUp().
Even here, It run FViewportClient::InputKey() regardless of MouseCaptureMode.

I think when it’s MouseCaptureMode::NoCapture, It shouldn’t run InputKey() for all mouse button inputs.