tdf#152073 qt: Draw background when drawing frame

For native drawing of a frame (border), use
the window background color as default color.

As mentioned in commit f39f21d92ec83c3a5062f29dd26214fc83012c06
("tdf#138010 (IV) VclScrolledWindow: Use actual border width"),
the Qt/KDE Breeze style uses a frame width of 2, with the
actual 1 pixel border being surrounded by a 1 pixel
padding/margin.

Transparent background did not ensure that the 1 pixel
padding/margin is repainted and could thus result
in artifacts from what was painted to that location
previously (s. the Basic IDE editor example from
tdf#152073).

Use the window background for the default image color
when drawing a frame to avoid this.

The gtk3 VCL plugin also explicitly draws a background
for the corresponding code path, using
`gtk_render_background`.

An alternative might be to explicitly draw the
background further up the call stack, or pass
an explicit background color from
`DecorationView::DrawFrame` and only apply
a default color in `QtGraphics_Controls::drawNativeControl`
when a background color (other than `COL_DEFAULT`)
is passed to that method.

Sample bt for gtk3:

    1   GtkSalGraphics::drawNativeControl     salnativewidgets-gtk.cxx 1835 0x7fffe6271f92
    2   SalGraphics::DrawNativeControl        salgdilayout.cxx         807  0x7fffee887739
    3   OutputDevice::DrawNativeControl       nativecontrols.cxx       287  0x7fffee58906e
    4   (anonymous namespace)::ImplDrawFrame  decoview.cxx             600  0x7fffee1b2055
    5   DecorationView::DrawFrame             decoview.cxx             886  0x7fffee1b32c8
    6   ImplSmallBorderWindowView::DrawWindow brdwin.cxx               742  0x7fffee0ea3ea
    7   ImplBorderWindow::Paint               brdwin.cxx               1630 0x7fffee0ee99a
    8   PaintHelper::DoPaint                  paint.cxx                313  0x7fffee0c9f1c
    9   vcl::Window::ImplCallPaint            paint.cxx                617  0x7fffee0cb4b3
    10  PaintHelper::~PaintHelper             paint.cxx                552  0x7fffee0cae14
    11  vcl::Window::ImplCallPaint            paint.cxx                623  0x7fffee0cb513

Many thanks to Rafael Lima for the
very helpful analysis in [1] and [2].

[1] https://bugs.documentfoundation.org/show_bug.cgi?id=152073#c2
[2] https://bugs.documentfoundation.org/show_bug.cgi?id=152073#c3

Change-Id: I08e6d05d0fc3d0e54952a65cd28dee92615df0a3
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/146419
Reviewed-by: Rafael Lima <rafael.palma.lima@gmail.com>
Tested-by: Jenkins
(cherry picked from commit 6f4b8b7cfe2907b7da46eec6951a0e09b836a6de)
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/146402
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
Reviewed-by: Michael Stahl <michael.stahl@allotropia.de>

1 files changed, 1 insertions, 0 deletions

diff --git a/vcl/qt5/QtGraphics_Controls.cxx b/vcl/qt5/QtGraphics_Controls.cxx
index e6e3d820da59..e08b84719e61 100644
--- a/vcl/qt5/QtGraphics_Controls.cxx
+++ b/vcl/qt5/QtGraphics_Controls.cxx
@@ -283,6 +283,7 @@ bool QtGraphics_Controls::drawNativeControl(ControlType type, ControlPart part,
                 break;
             }
             [[fallthrough]]; // QPalette::Window
+        case ControlType::Frame:
         case ControlType::Menubar:
         case ControlType::WindowBackground:
             m_image->fill(QApplication::palette().color(QPalette::Window).rgb());