Derive PlaceholderText color from Text color in QPalette

Until Qt 5.15, PlaceholderText color was automatically derived from the
Text color, dimmed by 50% (set alpha to 128, assuming the text color
is fully opaque).

This method has been adapted in the static qt_fusionPalette() method.
No other static method to populate palettes defines a PlaceholderText
color. As a consequence, the Text color is used to render placeholder
text characters.

This patch adds a static method, that sets missing PlaceholderText
colors by deriving them from the corresponding Text color.
It takes a dim factor by which the opacity is reduced versus the
corresponding Text color.

Fixes: QTBUG-105049
Change-Id: Iebb9d15d56f5d72d7ec68b7a0babdf5825b92665
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
(cherry picked from commit ebf733c6fb5503a48a0723d7c6b86a8e1bc60363)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/kernel/qpalette.cpp

@@ -60,6 +60,34 @@ static QColor qt_mix_colors(QColor a, QColor b)
                   (a.blue() + b.blue()) / 2, (a.alpha() + b.alpha()) / 2);
 }
 
+/*!
+    \internal
+
+    Derive undefined \l PlaceholderText colors from \l Text colors.
+    Unless already set, PlaceholderText colors will be derived from their Text pendents.
+    Colors of existing PlaceHolderText brushes will not be replaced.
+
+    \a alpha represents the dim factor as a percentage. By default, a PlaceHolderText color
+    becomes a 50% more transparent version of the corresponding Text color.
+*/
+static void qt_placeholder_from_text(QPalette &pal, int alpha = 50)
+{
+    if (alpha < 0 or alpha > 100)
+        return;
+
+    for (int cg = 0; cg < int(QPalette::NColorGroups); ++cg) {
+        const QPalette::ColorGroup group = QPalette::ColorGroup(cg);
+
+        // skip if the brush has been set already
+        if (!pal.isBrushSet(group, QPalette::PlaceholderText)) {
+            QColor c = pal.color(group, QPalette::Text);
+            const int a = (c.alpha() * alpha) / 100;
+            c.setAlpha(a);
+            pal.setColor(group, QPalette::PlaceholderText, c);
+        }
+    }
+}
+
 static void qt_palette_from_color(QPalette &pal, const QColor &button)
 {
     int h, s, v;
@@ -82,6 +110,8 @@ static void qt_palette_from_color(QPalette &pal, const QColor &button)
     pal.setColorGroup(QPalette::Disabled, buttonBrushDark, buttonBrush, buttonBrushLight150,
                       buttonBrushDark, buttonBrushDark150, buttonBrushDark,
                       whiteBrush, buttonBrush, buttonBrush);
+
+    qt_placeholder_from_text(pal);
 }
 
 /*!
@@ -569,6 +599,8 @@ QPalette::QPalette(const QBrush &windowText, const QBrush &button,
     init();
     setColorGroup(All, windowText, button, light, dark, mid, text, bright_text,
                   base, window);
+
+    qt_placeholder_from_text(*this);
 }
 
 
@@ -624,6 +656,8 @@ QPalette::QPalette(const QColor &button, const QColor &window)
     setColorGroup(Disabled, disabledForeground, buttonBrush, buttonBrushLight150,
                   buttonBrushDark, buttonBrushDark150, disabledForeground,
                   whiteBrush, baseBrush, windowBrush);
+
+    qt_placeholder_from_text(*this);
 }
 
 /*!