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How to firefox / chrome draw a scroll bar?

Scrolling in chrome or firefox has no handle. They are handleless, but they have the same characteristics and behavior as the default scrollbar. From this we can conclude that these browsers use the window API, for example DrawThemeBackground, to draw a scroll bar.

However, APIs like DrawThemeBackground, this is GDI, chrome / firefox uses skia / cario to render the entire canvas. My problem is how they combine this technology of two types?

Pesudo Code:

WM_PAINT:
    prepare canvas;
    draw the canvas with skia/cario;
    bitblt to the dc;
    draw the theme-related handless control;(???)
    bitblt to the dc or directly draw to the dc;(???)

Does the procedure do the same above?

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Firefox

In fact, cairo has a function to get DC from the surface of Cairo. Code example:

VOID OnPaint(HWND hwnd, HDC hdc)
{
    RECT rc;
    ::GetClientRect(hwnd, &rc);
    //draw one line
    cairo_surface_t* surface = cairo_win32_surface_create(hdc);
    cairo_t* cr = cairo_create(surface);
    cairo_set_source_rgb(cr, 0xff, 0, 0);
    cairo_set_line_width(cr, 1);
    cairo_move_to(cr, 0, 0);
    cairo_line_to(cr, rc.right, rc.bottom);
    cairo_stroke(cr);
    cairo_destroy(cr);

    //draw the theme background
    HDC hdcNew = cairo_win32_surface_get_dc(surface);
    HTHEME hTheme = OpenThemeData(NULL, L"SCROLLBAR");
    RECT rcArrow;
    SetRect(&rcArrow, 30, 30, 45, 45);
    DrawThemeBackground(hTheme, hdcNew, SBP_ARROWBTN, ABS_DOWNDISABLED, &rcArrow, NULL);
    cairo_surface_destroy(surface);
}

gfxWindowsNativeDrawing::BeginNativeDrawing() HDC gfxWindowsSurface::GetDCWithClip(gfxContext *ctx), , cairo win32.

Chrome

Skia skia canvas hdc, skia .

skia/ext/bitmap_platform_device_win.cc:

HDC BitmapPlatformDevice::BitmapPlatformDeviceData::GetBitmapDC() {
}

.

, , DC /skia canvas, .

:

void TestChromeExt(HWND hwnd, HDC hdc)
{
    RECT rc;
    GetClientRect(hwnd, &rc);
    skia::BitmapPlatformDevice* pBmpDevice = skia::BitmapPlatformDevice::Create(rc.right, rc.bottom, true);
    skia::PlatformCanvas *pCanvas = new skia::PlatformCanvas(pBmpDevice);
    pCanvas->clear(SK_ColorWHITE);
    SkPaint paint;
    paint.setColor(SK_ColorRED);
    paint.setStrokeWidth(3);
    paint.setStyle(SkPaint::kStroke_Style);
    pCanvas->drawLine(0, 0, rc.right, rc.bottom, paint);
    HDC memdc = skia::BeginPlatformPaint(pCanvas);
    RECT rcArrow;
    SetRect(&rcArrow, 100, 200, 120, 220);
    DrawThemeBackground(OpenThemeData(NULL, L"SCROLLBAR"), memdc, SBP_ARROWBTN, ABS_DOWNDISABLED, &rcArrow, NULL);
    skia::EndPlatformPaint(pCanvas);
    skia::DrawToNativeContext(pCanvas, hdc, 0, 0, &rc);
}

Windows, , , DrawThemeBackground. , ( ), DrawFrameControl .

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