Prevent current context from becoming inconsistent upon create()

Platform plugins have a tendency to make the newly created native
context current with a temporary surface. This is usually needed to
query some information related to the new context. Afterwards most of
them just reset to having nothing current.

This has two issues:

It unexpectedly changes the current context/surface. A call into
QOpenGLContext::create() does not imply that the current context will
get changed. This is the minor issue and we could probably live with
it (at least if it had been documented).

However, the real issue is that QOpenGLContext::currentContext() will
become inconsistent: it will still report whatever was current before
the create() even though on the EGL/WGL/GLX level that's not the case
anymore.

To prevent all this confusion the platform plugins can easily be
changed to restore whatever context/surface was current before they
altered it.

Change-Id: I6a5b4597c86571327524ddb13e0d02538593cc7b
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>

1 files changed, 4 insertions, 1 deletions

diff --git a/src/plugins/platforms/windows/qwindowsglcontext.cpp b/src/plugins/platforms/windows/qwindowsglcontext.cpp
index e09018ef69..1ed27f545d 100644
--- a/src/plugins/platforms/windows/qwindowsglcontext.cpp
+++ b/src/plugins/platforms/windows/qwindowsglcontext.cpp
@@ -1207,6 +1207,9 @@ QWindowsGLContext::~QWindowsGLContext()
 
 bool QWindowsGLContext::updateObtainedParams(HDC hdc, int *obtainedSwapInterval)
 {
+    HGLRC prevContext = wglGetCurrentContext();
+    HDC prevSurface = wglGetCurrentDC();
+
     if (!QOpenGLStaticContext::opengl32.wglMakeCurrent(hdc, m_renderingContext)) {
         qWarning("Failed to make context current.");
         return false;
@@ -1217,7 +1220,7 @@ bool QWindowsGLContext::updateObtainedParams(HDC hdc, int *obtainedSwapInterval)
     if (m_staticContext->wglGetSwapInternalExt && obtainedSwapInterval)
         *obtainedSwapInterval = m_staticContext->wglGetSwapInternalExt();
 
-    QOpenGLStaticContext::opengl32.wglMakeCurrent(0, 0);
+    QOpenGLStaticContext::opengl32.wglMakeCurrent(prevSurface, prevContext);
     return true;
 }