kcms/rules: add warning for low opacity

Once upon a time, in a moment of acute stupidity, I accidentally created a window rule making all windows completely transparent. I meant to go for 0% transparency when really the slider said 0% opacity. And once you've made the very window to change this setting invisible, the only way to fix that mistake is to log into a TTY or different DE and find the config file to edit manually. Since I've since seen several KRedditors fall into the same trap:  

Show a warning when active or inactive opacity is set to ~~<= 15%~~ < 25%.

![warning](/uploads/0b1178bf5782bff3b4293a0031bb4fca/warning.png)

src/kcms/rules/rulesmodel.cpp

@@ -247,6 +247,10 @@ QStringList RulesModel::warningMessages() const
                          m_rules["ignoregeometry"]->name());
     }
 
+    if (opacityWarning()) {
+        messages << i18n("Readability may be impaired with extremely low opacity values. At 0%, the window becomes invisible.");
+    }
+
     return messages;
 }
 
@@ -282,6 +286,21 @@ bool RulesModel::geometryWarning() const
     return (!ignoregeometry && (initialPos || initialSize || initialPlacement));
 }
 
+bool RulesModel::opacityWarning() const
+{
+    auto opacityActive = m_rules["opacityactive"];
+    const bool lowOpacityActive = opacityActive->isEnabled()
+        && opacityActive->policy() != Rules::Unused && opacityActive->policy() != Rules::DontAffect
+        && opacityActive->value().toInt() < 25;
+
+    auto opacityInactive = m_rules["opacityinactive"];
+    const bool lowOpacityInactive = opacityInactive->isEnabled()
+        && opacityActive->policy() != Rules::Unused && opacityActive->policy() != Rules::DontAffect
+        && opacityInactive->value().toInt() < 25;
+
+    return lowOpacityActive || lowOpacityInactive;
+}
+
 RuleSettings *RulesModel::settings() const
 {
     return m_settings;
@@ -577,15 +596,16 @@ void RulesModel::populateRuleList()
                                           QIcon::fromTheme("preferences-desktop-theme")));
     decocolor->setOptionsData(colorSchemesModelData());
 
-    addRule(new RuleItem(QLatin1String("opacityactive"),
-                         RulePolicy::ForceRule, RuleItem::Percentage,
-                         i18n("Active opacity"), i18n("Appearance & Fixes"),
-                         QIcon::fromTheme("edit-opacity")));
-
-    addRule(new RuleItem(QLatin1String("opacityinactive"),
-                         RulePolicy::ForceRule, RuleItem::Percentage,
-                         i18n("Inactive opacity"), i18n("Appearance & Fixes"),
-                         QIcon::fromTheme("edit-opacity")));
+    auto opacityactive = addRule(new RuleItem(QLatin1String("opacityactive"),
+                                              RulePolicy::ForceRule, RuleItem::Percentage,
+                                              i18n("Active opacity"), i18n("Appearance & Fixes"),
+                                              QIcon::fromTheme("edit-opacity")));
+    opacityactive->setFlag(RuleItem::AffectsWarning);
+    auto opacityinactive = addRule(new RuleItem(QLatin1String("opacityinactive"),
+                                                RulePolicy::ForceRule, RuleItem::Percentage,
+                                                i18n("Inactive opacity"), i18n("Appearance & Fixes"),
+                                                QIcon::fromTheme("edit-opacity")));
+    opacityinactive->setFlag(RuleItem::AffectsWarning);
 
     auto fsplevel = addRule(new RuleItem(QLatin1String("fsplevel"),
                                          RulePolicy::ForceRule, RuleItem::Option,

src/kcms/rules/rulesmodel.h

@@ -91,6 +91,7 @@ private:
 
     bool wmclassWarning() const;
     bool geometryWarning() const;
+    bool opacityWarning() const;
 
     static const QHash<QString, QString> x11PropertyHash();
     void updateVirtualDesktops();