pdf: text that names no colour comes out black, not the palette

A QTextDocument takes its default text colour from the application
palette, and markdown names no colour at all: in a window running a dark
palette -- which is what a phone in dark mode hands Qt -- every plain
paragraph was laid out #eeeeee on a white page. Printed, that is a blank
document; on screen it reads as "the font is oddly faint". The first
PDFs off the phone were exactly that.

- PdfTools: paperDocument() sets the default stylesheet (body colour
  #000000) before setHtml, and writePdf() lays that out. A colour the
  document does name still wins, so an .html handed to the tool keeps its
  own -- checked with a #cc0000 document under both palettes.
- --uitest: draws that document on a white image under a dark palette and
  looks for ink (238 before the fix, 0 after).

Verified on the NX789J: md_to_pdf on the device now writes CID TrueType
NotoSansCJKHK-Regular/-Bold text whose page renders at min=0.

app/src/core/PdfTools.cpp

@@ -35,6 +35,7 @@
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
+#include <memory>
 
 #include "core/AgentCore.h"
 #include "core/ToolProvider.h"
@@ -168,10 +169,9 @@ bool sourceHtml(const cst_tool_ctx_t *ctx, const ctl_tool_call_t *c,
  * strip. */
 bool writePdf(const QString &html, const QString &path, const QString &title,
               QString *err) {
-  QTextDocument doc;
-  doc.setHtml(html);
+  std::unique_ptr<QTextDocument> doc(paperDocument(html));
   if (!title.isEmpty())
-    doc.setMetaInformation(QTextDocument::DocumentTitle, title);
+    doc->setMetaInformation(QTextDocument::DocumentTitle, title);
 
   QPdfWriter writer(path);
   writer.setPageSize(QPageSize(QPageSize::A4));
@@ -186,17 +186,17 @@ bool writePdf(const QString &html, const QString &path, const QString &title,
     return false;
   }
   const QRectF page = writer.pageLayout().paintRectPixels(writer.resolution());
-  doc.setPageSize(page.size());
-  const int pages = qMax(1, doc.pageCount());
+  doc->setPageSize(page.size());
+  const int pages = qMax(1, doc->pageCount());
   for (int i = 0; i < pages; i++) {
     if (i)
       writer.newPage();
     painter.save();
     painter.translate(page.topLeft());
     painter.translate(0, -i * page.height());
-    doc.drawContents(&painter,
-                     QRectF(0, 0, page.width(), page.height())
-                         .translated(0, i * page.height()));
+    doc->drawContents(&painter,
+                      QRectF(0, 0, page.width(), page.height())
+                          .translated(0, i * page.height()));
     painter.restore();
   }
   painter.end();
@@ -285,6 +285,24 @@ int mdToPdfRun(const cst_tool_ctx_t *ctx, const ctl_tool_call_t *c,
 
 }  // namespace
 
+/* The document writePdf lays out, with a colour filled in for text that names
+ * none of its own.
+ *
+ * Paper is white and has no theme, but a QTextDocument takes its default text
+ * colour from the application palette: in a window running a dark palette --
+ * which is what a phone in dark mode hands Qt -- every plain markdown
+ * paragraph came out #eeeeee on a white page, i.e. a PDF that looks blank
+ * when printed, and reads as "the font is oddly faint" on screen. Naming the
+ * colour makes the page depend on the paper rather than on the window. A
+ * colour the document does name still wins (an .html file somebody handed us
+ * keeps its own), so this only fills the holes. */
+QTextDocument *paperDocument(const QString &html) {
+  auto *doc = new QTextDocument;
+  doc->setDefaultStyleSheet(QStringLiteral("body { color: #000000; }"));
+  doc->setHtml(html);
+  return doc;
+}
+
 int registerQtTools(AgentCore *core) {
   if (!core)
     return 0;

app/src/core/ToolProvider.h

@@ -19,6 +19,9 @@
 /* The engine wrapper of the front end (core/AgentCore.h). Only a pointer
  * travels through this header, so the class stays incomplete here. */
 class AgentCore;
+/* Same for the document md_to_pdf lays out (PdfTools.cpp): the caller owns
+ * it, and the test in ui/UiTest.cpp inspects how it draws. */
+class QTextDocument;
 
 namespace app {
 
@@ -48,6 +51,10 @@ int registerPlatformTools(AgentCore *core);
  * (they are Qt, not a platform API): today md_to_pdf, owned by "app:qt".
  * Called next to registerPlatformTools(). */
 int registerQtTools(AgentCore *core);
+/* The document md_to_pdf lays out from `html`: the page's own defaults
+ * applied (text with no colour of its own is black, not the window's palette
+ * colour). The caller owns the result. */
+QTextDocument *paperDocument(const QString &html);
 /* Drop them again (they belong to the owners "app:*"). */
 void unregisterPlatformTools(AgentCore *core);
 

app/src/ui/UiTest.cpp

@@ -13,6 +13,7 @@
 #include <QBuffer>
 #include <QFile>
 #include <QImage>
+#include <QPalette>
 #include <QJsonDocument>
 #include <QJsonObject>
 #include <QPainter>
@@ -22,14 +23,17 @@
 #include <QQmlContext>
 #include <QQuickWindow>
 #include <QTest>
+#include <QTextDocument>
 
 #include <QVariantMap>
 
 #include <cstdio>
 #include <cstring>
+#include <memory>
 
 #include "cst_agent.h"
 #include "cst_attach.h"
+#include "core/ToolProvider.h"
 #include "ui/SessionImages.h"
 #include "ui/ThemeIcons.h"
 
@@ -482,6 +486,36 @@ int runUiTest(AgentCore &core, const QString &size) {
     }
   }
 
+  /* Paper is white whatever the window's palette says: markdown carries no
+   * colour, so a dark window turned every plain paragraph #eeeeee -- a PDF
+   * that looked blank. Draw it on a white image and look for ink. */
+  {
+    const QPalette was = QGuiApplication::palette();
+    QPalette darkPalette = was;
+    darkPalette.setColor(QPalette::Text, QColor(0xee, 0xee, 0xee));
+    darkPalette.setColor(QPalette::WindowText, QColor(0xee, 0xee, 0xee));
+    QGuiApplication::setPalette(darkPalette);
+    std::unique_ptr<QTextDocument> page(app::paperDocument(
+        QStringLiteral("<html><body>text that names no colour of its own"
+                       "</body></html>")));
+    QImage paper(320, 60, QImage::Format_RGB32);
+    paper.fill(0xffffffffu);
+    QPainter ink(&paper);
+    page->drawContents(&ink);
+    ink.end();
+    int darkest = 255;
+    for (int y = 0; y < paper.height(); y++) {
+      const QRgb *row = reinterpret_cast<const QRgb *>(paper.constScanLine(y));
+      for (int x = 0; x < paper.width(); x++)
+        darkest = qMin(darkest, qGray(row[x]));
+    }
+    check(darkest < 64,
+          QStringLiteral("a dark window still lays out black ink on paper "
+                         "(darkest pixel %1)")
+              .arg(darkest));
+    QGuiApplication::setPalette(was);
+  }
+
   /* new sessions: the engine moves, the view empties, the store grows */
   const int before = core.sessions().size();
   clickMouse(win, QStringLiteral("newSessionButton"));