就是在这个地方啦，

HRESULT CCandidateListUIPresenter::_StartCandidateList(TfClientId tfClientId, _In_ ITfDocumentMgr *pDocumentMgr,
                                                       _In_ ITfContext *pContextDocument, TfEditCookie ec,
                                                       _In_ ITfRange *pRangeComposition, UINT wndWidth)
{
    pDocumentMgr;
    tfClientId;

    HRESULT hr = E_FAIL;

    if (FAILED(_StartLayout(pContextDocument, ec, pRangeComposition)))
    {
        goto Exit;
    }

    BeginUIElement();

    hr = MakeCandidateWindow(pContextDocument, wndWidth);
    if (FAILED(hr))
    {
        goto Exit;
    }

    Show(_isShowMode);

    RECT rcTextExt;
    if (SUCCEEDED(_GetTextExt(&rcTextExt)))
    {
        _LayoutChangeNotification(&rcTextExt);
    }

Exit:
    if (FAILED(hr))
    {
        _EndCandidateList();
    }
    return hr;
}

就是这里的 _GetTextExt，然后，我们再看 _GetTextExt 的函数体，

HRESULT CTfTextLayoutSink::_GetTextExt(_Out_ RECT *lpRect)
{
    HRESULT hr = S_OK;
    BOOL isClipped = TRUE;
    ITfContextView *pContextView = nullptr;

    hr = _pContextDocument->GetActiveView(&pContextView);
    if (FAILED(hr))
    {
        return hr;
    }

    if (FAILED(hr = pContextView->GetTextExt(_tfEditCookie, _pRangeComposition, lpRect, &isClipped)))
    {
        return hr;
    }

    pContextView->Release();

    return S_OK;
}

可以看到，最终是 pContextView→GetTextExt 这里。