tdf#93482 vcl rendercontext: add Window::RequestDoubleBuffering()

This allows applications to request enabling/disabling of
double-buffering of their VCL frame and all its children. It works
after-the-fact, too: so in case the start center creates the frame and
later that frame is reused for Writer, then Writer can turn on
double-buffering, still.

From a user's point of view, this means that next to
VCL_DOUBLEBUFFERING_FORCE_ENABLE, there is now also a
VCL_DOUBLEBUFFERING_ENABLE environment variable that enables a safe
subset that is not supposed to draw directly at all. Enable this for
Writer only, for now.

Change-Id: Ie2cbf7d467eae2cee37fb58a1efc0a8984204408

include/vcl/window.hxx

@@ -913,6 +913,8 @@ public:
 
     /// Can the widget derived from this Window do the double-buffering via RenderContext properly?
     bool                                SupportsDoubleBuffering() const;
+    /// Enable/disable double-buffering of the frame window and all its children.
+    void                                RequestDoubleBuffering(bool bRequest);
 
     void                                EnableAllResize( bool bEnable = true );
 

sw/source/uibase/uiview/view.cxx

@@ -719,6 +719,10 @@ SwView::SwView( SfxViewFrame *_pFrame, SfxViewShell* pOldSh )
     m_bIsPreviewDoubleClick(false),
     m_bAnnotationMode(false)
 {
+    static bool bRequestDoubleBuffering = getenv("VCL_DOUBLEBUFFERING_ENABLE");
+    if (bRequestDoubleBuffering)
+        m_pEditWin->RequestDoubleBuffering(true);
+
     // According to discussion with MBA and further
     // investigations, no old SfxViewShell will be set as parameter <pOldSh>,
     // if function "New Window" is performed to open an additional view beside
@@ -1056,7 +1060,13 @@ SwView::~SwView()
     m_pTogglePageBtn.disposeAndClear();
     delete m_pGlosHdl;
     delete m_pViewImpl;
+
+    // If this was enabled in the ctor for the frame, then disable it here.
+    static bool bRequestDoubleBuffering = getenv("VCL_DOUBLEBUFFERING_ENABLE");
+    if (bRequestDoubleBuffering)
+        m_pEditWin->RequestDoubleBuffering(false);
     m_pEditWin.disposeAndClear();
+
     delete m_pFormatClipboard;
 }
 

vcl/source/window/window.cxx

@@ -1075,7 +1075,7 @@ void Window::ImplInit( vcl::Window* pParent, WinBits nStyle, SystemParentData* p
         mpWindowImpl->mpFrameData->maResizeIdle.SetDebugName( "vcl::Window maResizeIdle" );
         mpWindowImpl->mpFrameData->mbInternalDragGestureRecognizer = false;
         if (!(nStyle & WB_DEFAULTWIN) && mpWindowImpl->mbDoubleBufferingRequested)
-            mpWindowImpl->mpFrameData->mpBuffer = VclPtrInstance<VirtualDevice>();
+            RequestDoubleBuffering(true);
         mpWindowImpl->mpFrameData->mbInBufferedPaint = false;
 
         if ( pRealParent && IsTopWindow() )
@@ -3908,6 +3908,18 @@ bool Window::SupportsDoubleBuffering() const
     return mpWindowImpl->mpFrameData->mpBuffer;
 }
 
+void Window::RequestDoubleBuffering(bool bRequest)
+{
+    if (bRequest)
+    {
+        mpWindowImpl->mpFrameData->mpBuffer = VclPtrInstance<VirtualDevice>();
+        // Make sure that the buffer size matches the frame size.
+        mpWindowImpl->mpFrameData->mpBuffer->SetOutputSizePixel(mpWindowImpl->mpFrameWindow->GetOutputSizePixel());
+    }
+    else
+        mpWindowImpl->mpFrameData->mpBuffer.reset();
+}
+
 /*
  * The rational here is that we moved destructors to
  * dispose and this altered a lot of code paths, that