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fix(xsettings): keep fractional DPI in Gdk/UnscaledDPI #1436

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fix(xsettings): keep fractional DPI in Gdk/UnscaledDPI #1436

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12 changes: 10 additions & 2 deletions src/xsettings/settingmanager.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -110,9 +110,17 @@ void SettingManager::setDoubleClickInterval(int interval)

void SettingManager::setGlobalScale(qreal scale)
{
const int windowScale = qMax(1, qFloor(scale));

m_resource->setPropertyValue(XResource::toByteArray(XResource::Xft_DPI), scale * BASE_DPI);
m_settings->setPropertyValue(XSettings::toByteArray(XSettings::Gdk_WindowScalingFactor), qFloor(scale));
m_settings->setPropertyValue(XSettings::toByteArray(XSettings::Gdk_UnscaledDPI), XSETTINGS_BASE_DPI_FIXED);
m_settings->setPropertyValue(XSettings::toByteArray(XSettings::Gdk_WindowScalingFactor), windowScale);
// Gdk/UnscaledDPI is defined as the DPI without the integer window scaling
// factor, but GDK/GTK (and Chromium through GTK's display config) prefer it
// over Xft/DPI. Keep the fractional part in it, otherwise GTK and Electron/
// Chromium clients resolve to the base 96 DPI and ignore Xft/DPI entirely,
// so fractional scales end up rendered at 100%.
m_settings->setPropertyValue(XSettings::toByteArray(XSettings::Gdk_UnscaledDPI),

@zzxyb zzxyb Sep 23, 2026

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Gdk_UnscaledDPI不就是96x1024吗？我没看懂你这个改动的作用

@LFRon LFRon Sep 23, 2026

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GNOME 的约定确实如此（我抓了 gsd 源码，plugins/xsettings/gsd-xsettings-manager.c:670-673）：

settings->window_scale = get_window_scale (manager);          /* 整数缩放（mutter UiScalingFactor） */settings->dpi          = dpi * 1024;                          /* Gdk/UnscaledDPI = 96×1024 */settings->scaled_dpi   = dpi * settings->window_scale * 1024; /* Xft/DPI = 96×scale×1024 */

treeland 的 ac2b0abcc 就是照这个抄的。但这个约定成立有个前提：合成器自己把 X11 表面整体放大（mutter 的 framebuffer scaling）——分数部分由合成器渲染时放大（略糊），XSETTINGS 只需要表达"整数窗口缩放"。

而 treeland/wlroots 没有这个能力：vendored wlroots 的 xwayland/ 里没有任何 scale 处理、公开头文件无 scale API、waylib 也不缩放 X11 表面（都已 grep 确认）。本机实测也印证：打了补丁后 X11 客户端恰好是 1.25×，如果是"合成器 1.25 + 提示 1.25"会变成 1.5625×。

所以在 treeland 上分数缩放只能靠客户端 DPI 提示表达，而 GDK/GTK 的取值顺序是"有 Gdk/UnscaledDPI 就优先用它当 DPI"（实测：live 上 UnscaledDPI=98304 + Xft/DPI=122880 → gtk-xft-dpi=98304；Xvfb 上把 UnscaledDPI 去掉 → 122880），Gdk/WindowScalingFactor 只承载整数部分。于是固定 96×1024 会把 0.25 直接丢掉：

客户端 结果
GTK gtk-xft-dpi=98304 → 96dpi → 100%
Chromium/Electron DSF = max(1, gdk_monitor_get_scale_factor()) × gtk-xft-dpi/1024/96 = 1×1.0 → 100%（Chromium 源码 ui/gtk/gtk_ui.cc + GtkUiPlatformX11::IncludeFontScaleInDeviceScale()==true；真 Electron 实测 1200x800 vs 1500x1000）
Qt 直接读 Xft/DPI=122880 → 125%（所以"有些应用正常"）

补丁的策略因此是：整数档完全遵循 GNOME 约定（100% → 98304；200% → 98304 + WSF=2；实测若删掉 UnscaledDPI，GDK 会拿 Xft/DPI=196608 当分辨率再乘 WSF=2 → 4.0× 双重放大，所以不能简单删键）；只有分数档把分数因子折进 Gdk/UnscaledDPI（= 有效 DPI ÷ 整数窗口缩放，125% → 122880）。这正是 treeland 在 ac2b0abcc 之前的原行为（scale × 98304）——是那个提交把分数档一起改成固定基值才引入了差异。

qRound(scale / windowScale * XSETTINGS_BASE_DPI_FIXED));
m_settings->setPropertyValue(XSettings::toByteArray(XSettings::Xft_DPI), qRound(scale * XSETTINGS_BASE_DPI_FIXED));
}

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