Qt: Handle dark mode in syntax highlighting.

Add ColorUtils::contrastingTextColor, which chooses an appropriate text
color from the current application palette for a given background.

Use it in SyntaxLineEdit and FontColorPreferencesFrame for each state
background color.

Fixes #15840.

ui/qt/font_color_preferences_frame.cpp

@@ -256,32 +256,35 @@ void FontColorPreferencesFrame::updateWidgets()
     //
     // Sample valid filter
     //
+    QColor ss_bg = ColorUtils::fromColorT(prefs_get_color_value(pref_valid_bg_, pref_stashed));
     ui->validFilterBGPushButton->setStyleSheet(color_button_ss.arg(
                                                    ColorUtils::fromColorT(prefs_get_color_value(pref_valid_bg_, pref_stashed)).name())
                                                    .arg(0));
     ui->validFilterSampleLineEdit->setStyleSheet(sample_text_ss.arg(
-                                                     "palette(text)",
-                                                     ColorUtils::fromColorT(prefs_get_color_value(pref_valid_bg_, pref_stashed)).name()));
+                                                     ColorUtils::contrastingTextColor(ss_bg).name(),
+                                                     ss_bg.name()));
 
     //
     // Sample invalid filter
     //
+    ss_bg = ColorUtils::fromColorT(prefs_get_color_value(pref_invalid_bg_, pref_stashed));
     ui->invalidFilterBGPushButton->setStyleSheet(color_button_ss.arg(
                                                      ColorUtils::fromColorT(prefs_get_color_value(pref_invalid_bg_, pref_stashed)).name())
                                                      .arg(0));
     ui->invalidFilterSampleLineEdit->setStyleSheet(sample_text_ss.arg(
-                                                       "palette(text)",
-                                                       ColorUtils::fromColorT(prefs_get_color_value(pref_invalid_bg_, pref_stashed)).name()));
+                                                       ColorUtils::contrastingTextColor(ss_bg).name(),
+                                                       ss_bg.name()));
 
     //
     // Sample warning filter
     //
+    ss_bg = ColorUtils::fromColorT(prefs_get_color_value(pref_deprecated_bg_, pref_stashed));
     ui->deprecatedFilterBGPushButton->setStyleSheet(color_button_ss.arg(
                                                         ColorUtils::fromColorT(prefs_get_color_value(pref_deprecated_bg_, pref_stashed)).name())
                                                         .arg(0));
     ui->deprecatedFilterSampleLineEdit->setStyleSheet(sample_text_ss.arg(
-                                                          "palette(text)",
-                                                          ColorUtils::fromColorT(prefs_get_color_value(pref_deprecated_bg_, pref_stashed)).name()));
+                                                          ColorUtils::contrastingTextColor(ss_bg).name(),
+                                                          ss_bg.name()));
 }
 
 void FontColorPreferencesFrame::changeColor(pref_t *pref)

ui/qt/utils/color_utils.cpp

@@ -179,6 +179,15 @@ QString ColorUtils::themeLinkStyle()
     return link_style;
 }
 
+const QColor ColorUtils::contrastingTextColor(const QColor color)
+{
+    bool background_is_light = color.lightness() > 127;
+    if ( (background_is_light && !ColorUtils::themeIsDark()) || (!background_is_light && ColorUtils::themeIsDark()) ) {
+        return QApplication::palette().text().color();
+    }
+    return QApplication::palette().base().color();
+}
+
 /*
  * Editor modelines
  *

ui/qt/utils/color_utils.h

@@ -52,8 +52,24 @@ public:
      * @return true if we're running in dark mode, false otherwise.
      */
     static bool themeIsDark();
+    /**
+     * Returns an appropriate link color for the current mode.
+     * @return A brush suitable for setting a text color.
+     */
     static QBrush themeLinkBrush();
+    /**
+     * Returns an appropriate HTML+CSS link style for the current mode.
+     * @return A "<style>a:link { color: ... ; }</style>" string
+     */
     static QString themeLinkStyle();
+    /**
+     * Returns either QPalette::Text or QPalette::Base as appropriate for the
+     * specified foreground color
+     *
+     * @param color The background color.
+     * @return A contrasting foreground color for the current mode / theme.
+     */
+    static const QColor contrastingTextColor(const QColor color);
 
 signals:
 

ui/qt/widgets/syntax_line_edit.cpp

@@ -25,6 +25,7 @@
 #include <ui/qt/utils/stock_icon.h>
 
 #include <QAbstractItemView>
+#include <QApplication>
 #include <QCompleter>
 #include <QKeyEvent>
 #include <QPainter>
@@ -41,13 +42,6 @@ SyntaxLineEdit::SyntaxLineEdit(QWidget *parent) :
     completion_model_(NULL),
     completion_enabled_(false)
 {
-    // Try to matche QLineEdit's placeholder text color (which sets the
-    // alpha channel to 50%, which doesn't work in style sheets).
-    // Setting the foreground color lets us avoid yet another background
-    // color preference and should hopefully make things easier to
-    // distinguish for color blind folk.
-    busy_fg_ = ColorUtils::alphaBlend(palette().text(), palette().base(), 0.5);
-
     setSyntaxState();
     setMaxLength(std::numeric_limits<quint32>::max());
 }
@@ -83,43 +77,59 @@ void SyntaxLineEdit::allowCompletion(bool enabled)
 
 void SyntaxLineEdit::setSyntaxState(SyntaxState state) {
     syntax_state_ = state;
+
+    QColor valid_bg = ColorUtils::fromColorT(&prefs.gui_text_valid);
+    QColor valid_fg = ColorUtils::contrastingTextColor(valid_bg);
+    QColor invalid_bg = ColorUtils::fromColorT(&prefs.gui_text_invalid);
+    QColor invalid_fg = ColorUtils::contrastingTextColor(invalid_bg);
+    QColor deprecated_bg = ColorUtils::fromColorT(&prefs.gui_text_deprecated);
+    QColor deprecated_fg = ColorUtils::contrastingTextColor(deprecated_bg);
+
+    // Try to matche QLineEdit's placeholder text color (which sets the
+    // alpha channel to 50%, which doesn't work in style sheets).
+    // Setting the foreground color lets us avoid yet another background
+    // color preference and should hopefully make things easier to
+    // distinguish for color blind folk.
+    QColor busy_fg = ColorUtils::alphaBlend(QApplication::palette().text(), QApplication::palette().base(), 0.5);
+
     state_style_sheet_ = QString(
             "SyntaxLineEdit[syntaxState=\"%1\"] {"
-            "  color: %5;"
-            "  background-color: %7;"
-            "}"
-
-            "SyntaxLineEdit[syntaxState=\"%2\"] {"
-            "  color: %5;"
-            "  background-color: %8;"
-            "}"
-
-            "SyntaxLineEdit[syntaxState=\"%3\"] {"
-            "  color: %5;"
-            "  background-color: %9;"
+            "  color: %2;"
+            "  background-color: %3;"
             "}"
 
             "SyntaxLineEdit[syntaxState=\"%4\"] {"
-            "  color: %10;"
+            "  color: %5;"
             "  background-color: %6;"
             "}"
+
+            "SyntaxLineEdit[syntaxState=\"%7\"] {"
+            "  color: %8;"
+            "  background-color: %9;"
+            "}"
+
+            "SyntaxLineEdit[syntaxState=\"%10\"] {"
+            "  color: %11;"
+            "  background-color: %12;"
+            "}"
             )
 
-            // CSS selectors
+            // CSS selector, foreground, background
             .arg(Valid)
+            .arg(valid_fg.name())
+            .arg(valid_bg.name())
+
             .arg(Invalid)
+            .arg(invalid_fg.name())
+            .arg(invalid_bg.name())
+
             .arg(Deprecated)
+            .arg(deprecated_fg.name())
+            .arg(deprecated_bg.name())
+
             .arg(Busy)
-
-            // Normal foreground / background
-            .arg("palette(text)")
-            .arg("palette(base)")
-
-            // Special foreground / background
-            .arg(ColorUtils::fromColorT(&prefs.gui_text_valid).name())
-            .arg(ColorUtils::fromColorT(&prefs.gui_text_invalid).name())
-            .arg(ColorUtils::fromColorT(&prefs.gui_text_deprecated).name())
-            .arg(busy_fg_.name())
+            .arg(busy_fg.name())
+            .arg(palette().base().color().name())
             ;
     setStyleSheet(style_sheet_);
 }

ui/qt/widgets/syntax_line_edit.h

@@ -70,7 +70,6 @@ private:
     QString state_style_sheet_;
     QString syntax_error_message_;
     QString token_chars_;
-    QColor busy_fg_;
     bool completion_enabled_;
 
 private slots: