Preserve an all-whitespace context value through save and reload

A property value that is entirely whitespace -- reported in #973 as a
workaround (setting a title-block custom variable to a single space, the
only way to give it a value other than blank before that bug was fixed in
#989) -- did not survive a save/reload cycle. Two independent causes, both
needed for the round trip to actually work:

1. DiagramContext::toXml() called .trimmed() on every stored value before
   writing it, unconditionally. For ordinary content this only strips
   accidental leading/trailing whitespace, but for a value that IS
   whitespace it collapses the entire thing to "", indistinguishable from
   a value that was never set.

2. QDomDocument::setContent(), used to parse the project file, discards a
   text node that is entirely whitespace by default. Confirmed in
   isolation, outside any QET code: parsing "<a> </a>" with the default
   ParseOptions gives QDomElement::text() == ""; adding
   ParseOption::PreserveSpacingOnlyNodes gives " ". So even once (1) stops
   destroying the value on save, the very next load throws it away again.

Fix (1) only trims when the trimmed result isn't empty, i.e. leaves an
all-whitespace value untouched. Fix (2) adds PreserveSpacingOnlyNodes to
the one setContent() call that parses a project file
(QETProject::readProjectXml()) -- not the other ~19 call sites in the
codebase (clipboard paste, element/macro loading, translations, autonum
context), which read different, narrower documents and are not implicated
in this report.

Blast radius of (2): every place that walks a QDomNode's children already
filters on isElement() (see QET::findInDomElement()), so the extra
whitespace-only text-node siblings this keeps around are inert wherever
current code already expected only elements. The one place it isn't inert
is exactly the bug -- calling .text() on an element whose entire content
is whitespace.

Verified end-to-end, not just at one stage: a single-space title-block
variable now survives two successive --resave cycles unchanged (confirmed
byte-for-byte in the saved XML), and renders as blank space rather than
literal placeholder text or a vanished value. Re-saved all 24 shipped
examples with and without this change and diffed: 23 byte-identical, the
one that differs (schema_indus.qet) differs only in element uuids -- and
resaving it twice with the SAME unpatched binary produces that same kind
of diff, confirming it is pre-existing non-determinism in files that
predate persisted uuids, unrelated to this change. Qt 6.10.2, ctest 11/11.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

sources/diagramcontext.cpp

@@ -151,8 +151,8 @@ bool DiagramContext::operator!=(const DiagramContext &dc) const
 void DiagramContext::toXml(QDomElement &e, const QString &tag_name) const
 {
 	foreach (QString key, keys()) {
-		if ((tag_name == "elementInformation") &&
+		const QString raw = m_content[key].toString();
-			(m_content[key].toString().trimmed().isEmpty())) {
+		if ((tag_name == "elementInformation") && raw.trimmed().isEmpty()) {
 			continue;
 		}
 		QDomElement property = e.ownerDocument().createElement(tag_name);
@@ -161,7 +161,15 @@ void DiagramContext::toXml(QDomElement &e, const QString &tag_name) const
 		property.removeAttribute("name");
 		property.setAttribute("show", m_content_show[key]);
 		property.setAttribute("name", key);
-		QDomText value = e.ownerDocument().createTextNode(m_content[key].toString().trimmed());
+		// Trim stray leading/trailing whitespace around real content, but
+		// not a value that IS whitespace: unconditionally trimming an
+		// all-whitespace string collapses it to "", which is silently
+		// indistinguishable from a value that was never set. A title-block
+		// custom variable set to a single space -- a workaround for #973,
+		// where an unset variable renders as its own literal placeholder --
+		// would otherwise vanish on the very next save.
+		const QString stored = raw.trimmed().isEmpty() ? raw : raw.trimmed();
+		QDomText value = e.ownerDocument().createTextNode(stored);
 		property.appendChild(value);
 		e.appendChild(property);
 	}

sources/qetproject.cpp

@@ -334,7 +334,18 @@ QETProject::ProjectState QETProject::openFile(QFile *file)
 		//file without a persisted uuid derives its uuid from them.
 	const QByteArray content = file->readAll();
 	QDomDocument xml_project;
-	if (!xml_project.setContent(content))
+	// PreserveSpacingOnlyNodes: without it, a text node that is entirely
+	// whitespace -- e.g. a title-block custom variable deliberately set to
+	// a single space, the only way to give it a value other than blank
+	// (bugtracker #973) -- is silently dropped by Qt's default parsing,
+	// and QDomElement::text() then returns "" for it exactly as if it had
+	// never been set. Confirmed in isolation: <a> </a> parses to text()=="",
+	// this option makes it text()==" ". Every place in this codebase that
+	// walks a QDomNode's children already filters on isElement() (see
+	// QET::findInDomElement()), so the extra whitespace-only text nodes
+	// this keeps around are inert everywhere but the two elements that
+	// call .text() on themselves -- which is exactly where the bug was.
+	if (!xml_project.setContent(content, QDomDocument::ParseOption::PreserveSpacingOnlyNodes))
 	{
 		if(opened_here) {
 			file->close();