diff --git a/kcms/fonts/fonts.h b/kcms/fonts/fonts.h
--- a/kcms/fonts/fonts.h
+++ b/kcms/fonts/fonts.h
@@ -160,7 +160,6 @@
 private:
     bool isSaveNeeded() const override;
     bool isDefaults() const override;
-    QFont applyFontDiff(const QFont &fnt, const QFont &newFont, int fontDiffFlags);
     void setNearestExistingFonts();
 
     FontsSettings *m_settings;
diff --git a/kcms/fonts/fonts.cpp b/kcms/fonts/fonts.cpp
--- a/kcms/fonts/fonts.cpp
+++ b/kcms/fonts/fonts.cpp
@@ -42,7 +42,6 @@
 #include <KAboutData>
 #include <KLocalizedString>
 #include <KPluginFactory>
-#include <KFontDialog>
 #include <KWindowSystem>
 
 #include "../krdb/krdb.h"
@@ -656,52 +655,47 @@
 
 void KFonts::adjustAllFonts()
 {
-    QFont font = m_settings->font();
-    KFontChooser::FontDiffFlags fontDiffFlags;
-    int ret = KFontDialog::getFontDiff(font, fontDiffFlags, KFontChooser::NoDisplayFlags);
+    bool ok = false;
+    QFont font = QFontDialog::getFont(&ok, m_settings->font(), nullptr, i18n("Select Font"));
 
-    if (ret == KDialog::Accepted && fontDiffFlags) {
+    if (ok) {
         if (!m_settings->isImmutable("font")) {
-            m_settings->setFont(applyFontDiff(m_settings->font(), font, fontDiffFlags));
+            m_settings->setFont(font);
         }
         if (!m_settings->isImmutable("menuFont")) {
-            m_settings->setMenuFont(applyFontDiff(m_settings->menuFont(), font, fontDiffFlags));
+            m_settings->setMenuFont(font);
         }
         if (!m_settings->isImmutable("toolBarFont")) {
-            m_settings->setToolBarFont(applyFontDiff(m_settings->toolBarFont(), font, fontDiffFlags));
+            m_settings->setToolBarFont(font);
         }
         if (!m_settings->isImmutable("activeFont")) {
-            m_settings->setActiveFont(applyFontDiff(m_settings->activeFont(), font, fontDiffFlags));
+            m_settings->setActiveFont(font);
         }
         if (!m_settings->isImmutable("smallestReadableFont")) {
-            m_settings->setSmallestReadableFont(applyFontDiff(m_settings->smallestReadableFont(), font, fontDiffFlags));
+            m_settings->setSmallestReadableFont(font);
         }
-        const QFont adjustedFont = applyFontDiff(m_settings->fixed(), font, fontDiffFlags);
-        if (QFontInfo(adjustedFont).fixedPitch() && !m_settings->isImmutable("fixed")) {
-            m_settings->setFixed(adjustedFont);
-        }
-    }
-}
 
-QFont KFonts::applyFontDiff(const QFont &fnt, const QFont &newFont, int fontDiffFlags)
-{
-    QFont font(fnt);
-
-    if (fontDiffFlags & KFontChooser::FontDiffSize) {
-        font.setPointSizeF(newFont.pointSizeF());
-    }
-    if ((fontDiffFlags & KFontChooser::FontDiffFamily)) {
-        font.setFamily(newFont.family());
-    }
-    if (fontDiffFlags & KFontChooser::FontDiffStyle) {
-        font.setWeight(newFont.weight());
-        font.setStyle(newFont.style());
-        font.setUnderline(newFont.underline());
-        font.setStyleName(newFont.styleName());
+        // Try to find a fixed-width/monspace font from the font family the user selected, e.g.:
+        // "Noto Sans" -> "Noto Sans Mono", "Ubuntu" -> "Ubuntu Mono"
+        // we take the first one that comes along, e.g. "Noto Sans Mono" _not_ "Noto Sans Mono Blk"
+        QFontDatabase fdb;
+        const QStringList fontFamilies = fdb.families();
+        QString mono;
+        for (const QString &family : fontFamilies) {
+            // QString::endsWith() is cheaper than contains(), so try it first
+           if (family.startsWith(font.family())
+               && (family.endsWith(QLatin1String("Mono"))
+                   || family.contains(QLatin1String("Mono")))) {
+                mono = family;
+                break;
+           }
+        }
+        // fallback if we find nothing
+        QFont f = mono.isEmpty() ? QFontDatabase::systemFont(QFontDatabase::FixedFont) : QFont(mono);
+        if (!m_settings->isImmutable("fixed")) {
+            m_settings->setFixed(f);
+        }
     }
-
-    return font;
 }
 
 #include "fonts.moc"
-