Printed output is microscopic: editor's pixel font size is handed to a HighResolution QPrinter

[weedwhitesandwine]

What happens

Printing from Omawrite (Ctrl+P) produces a page whose text is far too small to
read — roughly 0.5 mm tall. Every other part of the print path works: the
dialog appears, the job spools, the page comes out. It's just microscopic.

Cause

Backend::printDocument() copies the editor document's default font onto the
document it prints:

QPrinter printer(QPrinter::HighResolution);   // 1200 dpi
...
rendered.setDefaultFont(m_document->defaultFont());

The editor sizes that font in pixels — Main.qml sets
font.pixelSize: win.editorFontPixelSize on the TextArea, so the
QTextDocument's default font carries a pixel size, not a point size.
A QPrinter::HighResolution device runs at 1200 dpi, so those pixels are
interpreted as printer dots: an 18 px font is laid out 18/1200 of an inch
tall, about 0.38 mm.

Point sizes are resolution-independent and scale correctly; pixel sizes are not.

Reproduction

Minimal repro of the same sequence, rendered to PDF instead of paper:

QFont f(QStringLiteral("iA Writer Mono S"));
f.setPixelSize(18);                       // what Main.qml sets
QPrinter printer(QPrinter::HighResolution);
printer.setOutputFormat(QPrinter::PdfFormat);
printer.setOutputFileName("out.pdf");
QTextDocument doc;
doc.setDefaultFont(f);
doc.setMarkdown("# Heading\n\nThe quick brown fox jumps over the lazy dog.\n");
doc.print(&printer);

Measured with pdftotext -bbox, the body words come out 1.40 pt tall
(~0.5 mm). Converting the font to a point size first gives 17.55 pt
(~6.2 mm) — a 12.5x difference.

Suggested fix

Convert the editor's pixel size to points at the screen's DPI before printing,
so the page matches what the writer was looking at:

QFont Backend::printFont(const QFont &editorFont, qreal screenDpi) {
    QFont font = editorFont;
    if (font.pixelSize() <= 0)
        return font;                      // already point-sized

    const qreal dpi = screenDpi > 0.0 ? screenDpi : 96.0;
    font.setPointSizeF(font.pixelSize() * 72.0 / dpi);
    return font;
}

called as:

const QScreen *screen = QGuiApplication::primaryScreen();
rendered.setDefaultFont(printFont(m_document->defaultFont(),
                                  screen ? screen->logicalDotsPerInchY() : 0.0));

Happy to open a PR if that approach looks right.

Environment

• omawrite 0.5.0 (Arch package), also reproduced against main @ 8f98892
• Qt 6.11.2, Arch Linux, Hyprland/Wayland
• Printer: Epson EW-M638T over IPP; also reproduces printing to PDF