fix(linux): read dark mode from the settings portal - #6072
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Included review availability: Your plan provides up to 10 included reviews per hour; 7 remain after this review. WalkthroughLinux theme monitoring now uses the freedesktop Settings portal. Monitoring starts from ChangesLinux theme monitoring
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: ⚪ Minimal · up to The PR consolidates Linux dark-mode detection and theme-change handling without any supplied actionable merge-blocking risk; it is merge-ready after normal checks and review. Sequence Diagram(s)sequenceDiagram
participant PortalSettings
participant ThemeMonitor
participant LinuxApplication
PortalSettings->>ThemeMonitor: SettingChanged signal
ThemeMonitor->>PortalSettings: Read color-scheme
PortalSettings-->>ThemeMonitor: Dark-mode state
ThemeMonitor->>LinuxApplication: SystemThemeChanged event
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✅ Passed checks (4 passed)
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Inline comments:
In `@v3/pkg/application/application_linux_dbus.go`:
- Around line 50-53: Update the portal read in the relevant appearance/settings
method to use CallWithContext with a short, bounded timeout instead of obj.Call,
ensuring the context is canceled and timeout errors follow the existing return
0, false fallback; preserve the Read arguments and surrounding behavior used by
EnvironmentManager.IsDarkMode and monitorThemeChanges.
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v3/pkg/application/application_linux.gov3/pkg/application/application_linux_dbus.gov3/pkg/application/application_linux_gtk3.gov3/pkg/application/linux_cgo.gov3/pkg/application/linux_cgo_gtk3.go
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- v3/pkg/application/application_linux_gtk3.go
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Description
On Linux, both
Env.IsDarkMode()and thelinux:SystemThemeChangedevent are unreliable, in a different way on each backend. This consolidates the two divergent implementations into one portal-backed pair inapplication_linux_dbus.go, shared by the GTK3 and GTK4 builds.This change has been running in the netbirdio/wails fork, where it was reviewed and merged as netbirdio/wails#4. I am submitting it upstream with no changes beyond the rebase and one review follow-up, noted below.
There is no existing issue for this. #4665 ([v3] Window Theme API: Dark/Light/System support) is adjacent but is a feature request for a new API, not this defect. Happy to open a bug issue if you would prefer it tracked.
Three distinct problems, all in the same subsystem:
1. GTK3 reports light on every desktop until the first signal arrives.
a.themeis only ever written by theSettingChangedhandler, so it is""at launch. An app started on a dark desktop comes up light and stays light until the user toggles their colour scheme. GTK4 already reads the portal directly and does not have this problem.2. GTK4 never starts its theme monitor at all.
The watcher is started from
(*linuxApp).init(_ *App, options Options). That method is not part of theplatformAppinterface (application.go), and the only.init(call sites in the repo are the zero-argument(*App).init()inapplication_production.goandapplication_debug.go. Nothing calls the two-argumentlinuxApp.init, so on GTK4linux:SystemThemeChangednever fires. It is now started fromrun(), which is how the GTK3 path already does it (application_linux_gtk3.go).3. The shared watcher matches the wrong namespace.
monitorThemeChangesfilters on GNOME'sorg.gnome.desktop.interfacerather than the standardisedorg.freedesktop.appearance, and trusts the signal body's value instead of re-reading the portal.What this changes
application_linux_dbus.gobecomes the single source of truth for both backends — it carries nogtk3build constraint (//go:build linux && cgo && !android && !server), so one copy serves both:isDarkMode()readsorg.freedesktop.appearance/color-schemefrom the Settings portal on demand. This is deliberately not cached: the value is correct whether or not the monitor is running, which matters given problem 2 above went unnoticed.portalColorScheme()returns(uint32, bool)so "portal unreachable" is distinguishable from "prefers light", and accepts both a doubly-wrappedv{v{u}}reply (portal v1Read) and a singly-wrappedv{u}one (ReadOneand some implementations). Rejecting either shape would silently report light, which is the failure this change exists to remove.monitorThemeChanges()filters on sender, object path, interface and member, then re-reads the portal rather than trusting the signal body, and de-duplicates on the resolved boolean.The per-backend copies (
isDarkModein both platform files,listenForSystemThemeChangesin the GTK4 file) and the now-unusedthemestruct field are removed.monitorThemeChangesis consequently the sole emitter oflinux:SystemThemeChanged, which makes thecase uint(events.Linux.SystemThemeChanged)arm inprocessApplicationEventunreachable in bothlinux_cgo.goandlinux_cgo_gtk3.go; those are deleted too. No Linux C source callsprocessApplicationEvent, so there is no remaining producer. Removing them also drops anEnv.IsDarkMode()call that ran on the GTK main thread.Review follow-up
portalColorSchemenow bounds itsSettings.ReadwithCallWithContextand a2s deadline. godbus's
Object.Callpassescontext.Background(), so anunresponsive portal would have blocked the caller indefinitely, and
Env.IsDarkMode()is reachable from application code on the main thread. Adeadline surfaces as
call.Err, which the existing fallback already treats as"no preference", so a hung portal now reports exactly what an absent one does.
The budget is wide enough only to cover D-Bus activating the portal on a cold
first call; a local round-trip is sub-millisecond.
One point for maintainers on WEP scope
I have filed this as a bug fix rather than a WEP, because every case it changes was returning a wrong answer. There is one observable behaviour change worth your call:
GTK4 previously emitted
SystemThemeChangedon every appearancecolor-schemesignal. It now emits only when the resolved dark/light value actually changes, sono-preference↔prefer-lighttransitions no longer produce an event. I believe this is safe because the event's entire payload is theisDarkModeboolean, so a suppressed event carried no information a consumer could act on. If you consider that public behaviour, say so and I will split it out or raise a WEP.Changelog entry
Added to
v3/UNRELEASED_CHANGELOG.mdunder## Fixed. That makesauto-changelog-v3.ymltake itsskip=truepath on merge rather thanauto-filling, so this is the wording that ships:
Happy to reword it if you would rather it read differently.
Type of change
No public API surface changes. Everything added is unexported, and both removed symbols (
listenForSystemThemeChanges, thethemefield) were unexported.How Has This Been Tested?
Windows and macOS are unaffected; every changed file is behind a
linuxbuild tag.Provenance of these results, and what has not been re-run
Being explicit about this, because it bears on how much the matrices below are worth:
The runtime and toolchain verification was carried out against a
v3.0.0-beta.3base while developing the fix, and the change then shipped in the fork offv3.0.0-beta.9as netbirdio#4. This branch is that same commit cherry-picked onto currentmaster. The cherry-pick is conflict-free, and of the five files, the three carrying the substance of the fix —application_linux_dbus.go,application_linux_gtk3.go,linux_cgo_gtk3.go— are byte-identical between the fork base andmaster;application_linux.goandlinux_cgo.goauto-merged against unrelated upstream edits.gofmt -lis clean on all five onmaster.The runtime matrices below were not re-run on the current
mastertree. If you would like them repeated against the tip before merge, say so and I will do that.Test matrix
Tested on three desktops covering three portal backends and both Linux backends, all Wayland sessions:
xdg-desktop-portal-hyprland1.4.1xdg-desktop-portal-gnome46.2xdg-desktop-portal-kde6.7.4Compile, vet and unit tests
Both tag combinations, on Linux:
gofmt -lclean on all five files. The onlyvetoutput is the pre-existingpossible misuse of unsafe.Pointerindialogs_linux.go, and the only build warnings are the pre-existing GTK4gdk_x11_*deprecations inlinux_cgo.c. No new warnings. Both variants were additionally compiled on all three distros above.Runtime: the bug, before the change
A real Wails app set to follow the system theme, launched on a dark desktop, then toggled light and back. On stock
v3.0.0-beta.3:Both failure modes reproduce on every desktop tested, which is what convinced me these are two independent defects rather than one desktop-specific quirk. GTK3 recovers after a toggle because it does call
monitorThemeChangesfromrun(); GTK4 never recovers because of problem 2.Runtime: after the change
Same procedure, same desktops:
Portal values observed
Worth recording, because the three backends disagree about how they express "light" and the fix has to treat all of them as not-dark:
org.freedesktop.appearancewhen lightuint32 0(no preference)uint32 1uint32 2(prefer light)uint32 1uint32 1All three also publish GNOME's
org.gnome.desktop.interfacecolor-schemeas a string. That is why dropping it from the signal filter is safe here: every backend tested serves the standardised namespace, so nothing depended on the GNOME-specific one.How the verdicts were measured
Light/dark was decided by counting pixels matching the app's known surface colours in each screenshot, not by overall image brightness. That matters: two runs were silently invalidated by the VM blanking its screen mid-test, and this method reports them as "no app surface found" instead of producing a confident wrong answer.
Reproducing the original bug
On a GTK3 build: set your desktop to prefer dark, launch an app that resolves its theme from
Env.IsDarkMode(), and observe it come up light. On a GTK4 build: launch it, then change your desktop colour scheme and observe that nothing happens. Confirm the desktop preference independently with:gdbus call --session --dest org.freedesktop.portal.Desktop \ --object-path /org/freedesktop/portal/desktop \ --method org.freedesktop.portal.Settings.Read \ org.freedesktop.appearance color-scheme # expect (<<uint32 1>>,) when the desktop prefers darkNot covered
mastertree at runtime — see the provenance note above.xdg-desktop-portalat all). The unreachable path is exercised only by inspection, where it returns light — matching the previous GTK4 behaviour.Test Configuration
wails3 doctoron the Arch host, where the build and vet runs above were done:The other two hosts ran Go 1.27.0 with their distro toolchains: Ubuntu 24.04.4
(gcc 13.3.0, GTK 3.24.41 / 4.14.5) and Fedora 44 (gcc 16.2.1, GTK 3.24.52 /
4.22.4). Between them the GTK4 runtimes exercised end-to-end are 4.14.5 and
4.22.4, eight releases apart, and the GTK3 runtimes are 3.24.41 and 3.24.52.
Note for anyone reproducing this on a VM
All three hosts are GPU-less QEMU guests, and GTK4 segfaults there under its default renderer — in
gtk_window_presenton Ubuntu (preceded byMESA: ZINK: failed to choose pdev) and ing_application_runon Fedora. This is unrelated to the change; the GTK4 runs above needed:Checklist:
website/src/pages/changelog.mdxwith details of this PR (v3 changelog entries are added automatically) — n/a, this is v3; entry added tov3/UNRELEASED_CHANGELOG.mdinstead, which suppresses the auto-fillOn the unchecked box: the fix is verified at runtime across the matrices above but not yet by an automated test.
isColorSchemeChangeis a pure function over a*dbus.Signaland is straightforwardly unit-testable without a session bus, alongside the existing*_linux_test.gofiles in this package. Happy to add that before merge — say the word.Licence and provenance
No third-party code is included in this change. It is offered under the project's MIT Licence. It was previously merged into the netbirdio/wails fork as netbirdio#4; I am the author of the contribution in both places.
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