Fix label mask sizes get reset to 0

This paper cut rears its head in that when first enabling
label masks for an existing project layer, the mask size will
default to "0", which is confusing for users as it seems to
have no effect.

The older maskSize attribute used to be treated as 0 instead
of the default 1.5 mm size when no mask settings were available, and
then when the project was saved this incorrect 0 value would
become an actual valid attribute in the XML.

Since we can't now differentiate a valid 0 value from an accidental
0 value in older projects, we instead assume "0" as a mistake and
reset it to 1.5. when the project is saved the newer maskSize2
attribute will be used and we know that a "0" value WAS an explicit
user choice.

src/core/textrenderer/qgstextmasksettings.cpp

@@ -205,7 +205,22 @@ void QgsTextMaskSettings::readXml( const QDomElement &elem )
   const QDomElement textMaskElem = elem.firstChildElement( QStringLiteral( "text-mask" ) );
   d->enabled = textMaskElem.attribute( QStringLiteral( "maskEnabled" ), QStringLiteral( "0" ) ).toInt();
   d->type = static_cast<QgsTextMaskSettings::MaskType>( textMaskElem.attribute( QStringLiteral( "maskType" ), QStringLiteral( "0" ) ).toInt() );
-  d->size = textMaskElem.attribute( QStringLiteral( "maskSize" ), QStringLiteral( "0" ) ).toDouble();
+  if ( textMaskElem.hasAttribute( QStringLiteral( "maskSize2" ) ) )
+  {
+    d->size = textMaskElem.attribute( QStringLiteral( "maskSize2" ), QStringLiteral( "1.5" ) ).toDouble();
+  }
+  else
+  {
+    // the older maskSize attribute used to be treated as 0 instead of the default 1.5 mm size when no mask
+    // settings were available, and then when the project was saved this incorrect 0 value would become an
+    // actual valid attribute in the XML. Since we can't now differentiate a valid 0 value from an accidental
+    // 0 value in older projects, we instead assume "0" as a mistake and reset it to 1.5.
+    // when the project is saved the newer maskSize2 attribute will be used and we know that a "0" value
+    // WAS an explicit user choice.
+    d->size = textMaskElem.attribute( QStringLiteral( "maskSize" ) ).toDouble();
+    if ( d->size == 0 )
+      d->size = 1.5;
+  }
   d->sizeUnit = QgsUnitTypes::decodeRenderUnit( textMaskElem.attribute( QStringLiteral( "maskSizeUnits" ) ) );
   d->sizeMapUnitScale = QgsSymbolLayerUtils::decodeMapUnitScale( textMaskElem.attribute( QStringLiteral( "maskSizeMapUnitScale" ) ) );
   d->joinStyle = static_cast< Qt::PenJoinStyle >( textMaskElem.attribute( QStringLiteral( "maskJoinStyle" ), QString::number( Qt::RoundJoin ) ).toUInt() );
@@ -224,6 +239,8 @@ QDomElement QgsTextMaskSettings::writeXml( QDomDocument &doc ) const
   textMaskElem.setAttribute( QStringLiteral( "maskEnabled" ), d->enabled );
   textMaskElem.setAttribute( QStringLiteral( "maskType" ), d->type );
   textMaskElem.setAttribute( QStringLiteral( "maskSize" ), d->size );
+  // deliberate -- see comment in readXml
+  textMaskElem.setAttribute( QStringLiteral( "maskSize2" ), d->size );
   textMaskElem.setAttribute( QStringLiteral( "maskSizeUnits" ), QgsUnitTypes::encodeUnit( d->sizeUnit ) );
   textMaskElem.setAttribute( QStringLiteral( "maskSizeMapUnitScale" ), QgsSymbolLayerUtils::encodeMapUnitScale( d->sizeMapUnitScale ) );
   textMaskElem.setAttribute( QStringLiteral( "maskJoinStyle" ), static_cast< unsigned int >( d->joinStyle ) );