tdf#130857 qt weld: Make opt-in for now

Except for the simple message dialogs created in
QtInstance::CreateMessageDialog, disable the use
of native Qt dialogs by default for now, and only
use those if the user explicitly opts in to use
them by setting the SAL_VCL_QT_USE_WELDED_WIDGETS
environment variable.

While those dialogs declared as supported in
QtInstanceBuilder::IsUIFileSupported are meant to be
fully functional, they are only a small subset
of all LO dialogs and not all properties for
widgets are evaluated by QtBuilder at this point in
time (e.g. not all spacing, margins, text formatting
properties,...).

Therefore, default to VclBuilder again for now in order
to have a more consistent visual appearance for dialogs
with the Qt-based VCL plugins.

This can be reconsidered in the future, but at least for
25.2, sticking to VclBuilder by default seems reasonable
to me.

Change-Id: I7b275d3d3759093a6680327faeb3b86d7c623cea
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/176013
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>

vcl/README.vars.md

@@ -68,7 +68,9 @@ will be used to write the log under `instdir/uitest/`.
 * `SAL_VCL_QT_NO_FONTCONFIG` - ignore fontconfig provided font substitutions
 * `SAL_VCL_QT_NO_NATIVE` - disable `QStyle`'d controls
 * `SAL_VCL_QT_USE_QFONT` - use `QFont` for text layout and rendering (default is to use cairo)
-* `SAL_VCL_QT_NO_WELDED_WIDGETS` - disable the use of welded Qt widgets, reverts to previous VCL widget behavior
+* `SAL_VCL_QT_USE_WELDED_WIDGETS` - enable use of welded Qt widgets also for dialogs created from .ui files
+* `SAL_VCL_QT_NO_WELDED_WIDGETS` - disable the use of welded Qt widgets completely (also for simple message
+  dialogs), reverts to previous VCL widget behavior
 
 ## Mac
 

vcl/qt5/QtInstance.cxx

@@ -830,7 +830,10 @@ QWidget* QtInstance::GetNativeParentFromWeldParent(weld::Widget* pParent)
 std::unique_ptr<weld::Builder>
 QtInstance::CreateBuilder(weld::Widget* pParent, const OUString& rUIRoot, const OUString& rUIFile)
 {
-    if (!QtData::noWeldedWidgets() && QtInstanceBuilder::IsUIFileSupported(rUIFile))
+    // for now, require explicitly enabling use of QtInstanceBuilder via SAL_VCL_QT_USE_WELDED_WIDGETS
+    static const bool bUseWeldedWidgets = (getenv("SAL_VCL_QT_USE_WELDED_WIDGETS") != nullptr);
+    if (bUseWeldedWidgets && !QtData::noWeldedWidgets()
+        && QtInstanceBuilder::IsUIFileSupported(rUIFile))
     {
         QWidget* pQtParent = GetNativeParentFromWeldParent(pParent);
         return std::make_unique<QtInstanceBuilder>(pQtParent, rUIRoot, rUIFile);