docs: SG-43585: record why macOS keeps the software Qt Quick backend

The QT_QUICK_BACKEND comments no longer described reality. Both files still
explained the setting as a Qt 5.12.1 workaround for QWebEngineView conflicting
with the QGLWidget viewport, which has not been the reason since the viewport
became a native QOpenGLWindow.

src/bin/apps/rv/main.cpp (Windows, Linux): drop the block outright. The call was
already commented out, so all that remained was a commented-out line plus a
rationale for a decision the code no longer makes. Linux has since confirmed the
setting is not needed there -- it launches and runs docked web panels without it.

src/bin/nsapps/RV/main.cpp (macOS): the setting stays, and the comment now says
why, because it is load bearing rather than vestigial. The viewport window is
embedded with QWidget::createWindowContainer, making it a QObject child of the
top level window's QWidgetWindow. With the hardware Qt Quick backend, adding a
QWebEngineView makes Qt destroy and recreate that native subtree; the viewport
window is deleted with it rather than reparented, and QWindowContainer then
dereferences the window it just lost, so RV segfaults inside Qt. Measured on
macOS with the four line reproducer now quoted in the comment: software backend
survives 2/2, hardware backend crashes 2/2.

The cost is that macOS web panels composite in software, so macOS gets the cheap
viewport repaint half of this work but not the hardware web panel half. Noted in
the comment so the asymmetry with Windows and Linux reads as deliberate.

Comment-only; no compiled behaviour changes.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Signed-off-by: Bernard Laberge <bernard.laberge@autodesk.com>

Author: bernie-laberge

src/bin/apps/rv/main.cpp

@@ -353,18 +353,6 @@ int utf8Main(int argc, char* argv[])
 
     TwkFB::ThreadPool::initialize();
 
-    // Qt 5.12.1 specific
-    // Disable Qt Quick hardware rendering because QwebEngineView conflicts with
-    // QGLWidget
-    //
-    // Direction C: the RV viewport is now a native GL window (GLWindow) hosted
-    // via createWindowContainer, not a QOpenGLWidget. The main window no longer
-    // forces the OpenGL RHI backend, so QtWebEngine's QQuickWidget can composite
-    // on the platform default backend at hardware speed. The old software Qt
-    // Quick workaround (which caused the ~77 ms per-frame web-view composite) is
-    // therefore no longer needed.
-    // setEnvVar("QT_QUICK_BACKEND", "software");
-
 #if defined(PLATFORM_LINUX)
     // Work around for Wacom Tablet issue on linux
     // Note: This is a Qt 5.12.4 regression

src/bin/nsapps/RV/main.cpp

@@ -284,9 +284,34 @@ int main(int argc, char* argv[])
 
     setPlatformSpecificLocale();
 
-    // Qt 5.12.1 specific
-    // Disable Qt Quick hardware rendering because QwebEngineView conflicts with
-    // QGLWidget
+    //
+    //  Keep Qt Quick on the software backend. Do not remove this on macOS.
+    //
+    //  It was originally added because QWebEngineView conflicted with the
+    //  QGLWidget viewport, and src/bin/apps/rv/main.cpp has since dropped it for
+    //  Windows and Linux -- but on macOS it is now load bearing for a different
+    //  reason. The viewport is a native QOpenGLWindow embedded via
+    //  QWidget::createWindowContainer, which makes it a QObject child of the top
+    //  level window's QWidgetWindow. With the hardware Qt Quick backend, adding a
+    //  QWebEngineView to the widget tree makes Qt destroy and recreate that
+    //  native subtree; the viewport window is deleted with it rather than
+    //  reparented, and QWindowContainer then dereferences the window it just
+    //  lost. RV segfaults inside Qt, before it can react.
+    //
+    //  Reproducible in four lines from any docked panel -- with the software
+    //  backend this survives, with the hardware backend it crashes:
+    //
+    //      w = rv.qtutils.sessionWindow()
+    //      d = QtWidgets.QDockWidget('t', w)
+    //      v = QtWebEngineWidgets.QWebEngineView(d)
+    //      d.setWidget(v); w.addDockWidget(QtCore.Qt.RightDockWidgetArea, d)
+    //      v.setHtml('<b>hi</b>')
+    //
+    //  The cost of keeping it is that web panels composite in software on macOS,
+    //  so macOS gets the cheap-viewport-repaint half of SG-43585 but not the
+    //  hardware web panel half. Removing it needs the viewport to stop being
+    //  owned by the top level window's native subtree, not just a comment change.
+    //
     setenv("QT_QUICK_BACKEND", "software", 0 /* changeFlag : Do not change the existing value */);
 
     // Prevent usage of native sibling widgets on Mac. This attribute can be