Re-add Force Font DPI spinbox on Wayland

People are using this feature to work around Wayland's terrible
fractional scaling implementation that makes everything super blurry. I
can't in good conscience tell them to use it anyway because it really
does look so bad. :( See https://bugs.kde.org/show_bug.cgi?id=433269.

This reverts commit 65defddee95db6738d5e2c80f6c7b373fde451a1
This reverts commit 40afa58ab9bf0d0808edb891f9cd855316a82411

BUG: 433115
FIXED-IN: 5.21.2

kcms/fonts/fonts.cpp

@@ -129,9 +129,10 @@ void KFonts::load()
 void KFonts::save()
 {
     auto dpiItem = fontsAASettings()->findItem("forceFontDPI");
+    auto dpiWaylandItem = fontsAASettings()->findItem("forceFontDPIWayland");
     auto antiAliasingItem = fontsAASettings()->findItem("antiAliasing");
-    Q_ASSERT(dpiItem && antiAliasingItem);
-    if (dpiItem->isSaveNeeded() || antiAliasingItem->isSaveNeeded()) {
+    Q_ASSERT(dpiItem && dpiWaylandItem && antiAliasingItem);
+    if (dpiItem->isSaveNeeded() || dpiWaylandItem->isSaveNeeded() || antiAliasingItem->isSaveNeeded()) {
         emit aliasingChangeApplied();
     }
 

kcms/fonts/fontsaasettings.cpp

@@ -264,6 +264,7 @@ FontsAASettings::FontsAASettings(QObject *parent)
     addItemInternal("subPixel", defaultSubPixel(), &FontsAASettings::subPixelChanged);
     addItemInternal("hinting", defaultHinting(), &FontsAASettings::hintingChanged);
 
+    connect(this, &FontsAASettings::forceFontDPIWaylandChanged, this, &FontsAASettings::dpiChanged);
     connect(this, &FontsAASettings::forceFontDPIChanged, this, &FontsAASettings::dpiChanged);
 }
 

kcms/fonts/fontsaasettingsbase.kcfg

@@ -5,6 +5,10 @@
       http://www.kde.org/standards/kcfg/1.0/kcfg.xsd" >
   <kcfgfile name="kcmfonts" />
   <group name="General">
+    <entry name="forceFontDPIWayland" type="UInt">
+      <label>Force font DPI Wayland</label>
+      <default>0</default>
+    </entry>
     <entry name="forceFontDPI" type="UInt">
       <label>Force font DPI on X11</label>
       <default>0</default>

kcms/fonts/package/contents/ui/main.qml

@@ -362,9 +362,6 @@ KCM.SimpleKCM {
 
             RowLayout {
                 Layout.preferredWidth: formLayout.maxImplicitWidth
-                // We don't want people messing with the font DPI on Wayland;
-                // they should always be using the global scaling system instead
-                visible: Qt.platform.pluginName === "xcb"
 
                 QtControls.CheckBox {
                     id: dpiCheckBox
@@ -375,6 +372,13 @@ KCM.SimpleKCM {
                         dpiTwiddledMessage.visible = checked
                     }
 
+                    // dpiSpinBox will set forceFontDPI or forceFontDPIWayland,
+                    // so only one SettingStateBinding will be activated at a time.
+                    KCM.SettingStateBinding {
+                        configObject: kcm.fontsAASettings
+                        settingName: "forceFontDPIWayland"
+                        extraEnabledConditions: antiAliasingCheckBox.checked && !kcm.fontsAASettings.isAaImmutable
+                    }
                     KCM.SettingStateBinding {
                         configObject: kcm.fontsAASettings
                         settingName: "forceFontDPI"
@@ -390,6 +394,13 @@ KCM.SimpleKCM {
                     to: 999
                     from: 1
 
+                    // dpiSpinBox will set forceFontDPI or forceFontDPIWayland,
+                    // so only one SettingStateBinding will be activated at a time.
+                    KCM.SettingStateBinding {
+                        configObject: kcm.fontsAASettings
+                        settingName: "forceFontDPIWayland"
+                        extraEnabledConditions: dpiCheckBox.enabled && dpiCheckBox.checked
+                    }
                     KCM.SettingStateBinding {
                         configObject: kcm.fontsAASettings
                         settingName: "forceFontDPI"

kcms/krdb/krdb.cpp

@@ -438,8 +438,13 @@ void runRdb(uint flags)
 
         int dpi;
 
+        //even though this sets up the X rdb, we want to use the value the
+        //user has set to use when under wayland - as X apps will be scaled by the compositor
         if (KWindowSystem::isPlatformWayland()) {
-            dpi = 96; // Don't change the default DPI at all under wayland
+            dpi = cfgfonts.readEntry("forceFontDPIWayland", 0);
+            if (dpi == 0) { //with wayland we want xwayland to run at 96 dpi (unless set otherwise) as we have wayland scaling on top
+                dpi = 96;
+            }
         } else {
             dpi = cfgfonts.readEntry("forceFontDPI", 0);
         }

startkde/startplasma-wayland.cpp

@@ -30,6 +30,14 @@ int main(int argc, char **argv)
     createConfigDirectory();
     setupCursor(true);
 
+    {
+        KConfig fonts(QStringLiteral("kcmfonts"));
+        KConfigGroup group = fonts.group("General");
+        auto dpiSetting = group.readEntry("forceFontDPIWayland", 96);
+        auto dpi = dpiSetting == 0 ? 96 : dpiSetting;
+        qputenv("QT_WAYLAND_FORCE_DPI", QByteArray::number(dpi));
+    }
+
     // Query whether org.freedesktop.locale1 is available. If it is, try to
     // set XKB_DEFAULT_{MODEL,LAYOUT,VARIANT,OPTIONS} accordingly.
     {