Linux (Qt 6)

linux-qt renders Qt 6 Widgets through a small compiled C++ shim. It’s the toolkit beneath KDE Plasma, its QAccessible layer bridges to the native accessibility API on every OS (no other Day backend does that today), and it is portable enough to also be a development target on macOS and Windows.

Home

Controls

Canvas & shapes

Tabs

Getting started

Qt 6 development packages and pkg-config are the requirement; the shim is built by build.rs, and no moc step is involved.

sudo apt install qt6-base-dev pkg-config      # Debian/Ubuntu
sudo dnf install qt6-qtbase-devel pkgconf     # Fedora
brew install qt pkg-config                    # macOS, for the macos-qt dev combo

cargo install day-cli
day new app my-app --toolkit linux-qt
cd my-app
day launch -p linux-qt

Two optional modules enable two pieces. Both degrade gracefully rather than failing the build:

Module	Enables	Without it
`qt6-webengine-dev` (`Qt6WebEngineWidgets`)	`webview()`	a URL label; navigation calls become no-ops
`qt6-multimedia-dev` (`Qt6MultimediaWidgets`)	`media()`	a URL label; play/pause become no-ops

day doctor --toolkit qt reports the whole set.

day pack -p linux-qt        # single-file .flatpak (org.kde.Platform runtime)

If your binary links QtWebEngine, day pack adds the Qt WebEngine BaseApp — about 87 MB of Chromium — because no runtime ships it. Apps that don’t link it stay small; that check is on the actual binary, not on a flag.

Caveats and special considerations

toggle is a QCheckBox, not a switch. Qt Widgets has no native switch control. This is the one place where the same Day code produces a visibly different control shape than on the other desktops.
The list is emulated. Day drives a QScrollArea with its own recycling rather than QListView, so very large lists behave differently than a model/view Qt app would.
Qt 6.8+ for forced light/dark. DAY_THEME uses QStyleHints::setColorScheme; on older Qt (Ubuntu 24.04 ships 6.4) Day falls back to the Fusion style with a hand-built dark palette.
No spell-check, because Qt ships none — Cap::TextSpellCheck reports unsupported and the patch is a no-op.
No Lottie, no map on this backend — each renders its placeholder. cover() presents as a topmost window-sized child (no transition, programmatic dismissal only).
Qt on macOS and Windows (macos-qt, windows-qt) is a development combo: no packaging. Windows uses the MSYS2/MinGW toolchain and is marked experimental in CI. MSYS2 has no Qt6 WebEngine, so the web view degrades there — and the MinGW linker currently drops external piece renderer registrations, so those pieces draw placeholders on windows-qt.

What each piece becomes

Day piece	Qt class
`column` / `row` / `stack` / `section`	`QWidget` (Day owns layout)
`label`	`QLabel`
`button`	`QPushButton`
`toggle`	`QCheckBox` — see the caveat above
`slider`	`QSlider`
`text_field`	`QLineEdit`
`text_area`	`QPlainTextEdit`
`picker().menu()`	`QComboBox`
`picker().segmented()`	checkable `QPushButton`s in an exclusive group
`picker().inline()`	`QRadioButton`s in an exclusive group
`progress(f)`	`QProgressBar`
`spinner()`	`QProgressBar` in busy mode (Qt has no spinner widget)
`divider`	`QFrame` (`HLine`)
`scroll`	`QScrollArea`
`list`	`QScrollArea` with Day-side recycling
`nav` (split)	`QSplitter`; the stack header adds a `QToolButton`
`nav_menu`	`QListWidget`
`tabs`	`QTabWidget`
`image`	`QLabel` subclass painting a scaled pixmap
`canvas`	`QWidget` subclass painting with `QPainter`
dialogs / alerts	`QMessageBox`
prompts	`QInputDialog`
file dialogs	`QFileDialog`
menu bar / context menus	`QMenuBar` / `QMenu`
external `activity()`	`QProgressBar` (busy)
external `combobox()`	editable `QComboBox`
external `search_field()`	`QLineEdit` with a clear button and a find icon
external `date_picker()`	`QDateEdit` with a calendar popup, or `QCalendarWidget` inline
external `time_picker()`	`QTimeEdit`
external `webview()`	`QWebEngineView`
external `media()`	`QMediaPlayer` + `QVideoWidget`

Text measurement asks QLabel for its natural width with wrapping temporarily off, then heightForWidth at the granted width; the text area uses QTextDocument. Everything else uses sizeHint().