feat: fade the account colour across a card

2 files changed, 46 insertions, 0 deletions

diff --git a/src/carddelegate.cpp b/src/carddelegate.cpp
index 9a98c69..6541932 100644
--- a/src/carddelegate.cpp
+++ b/src/carddelegate.cpp
@@ -26,6 +26,7 @@
 #include <QApplication>
 #include <QDateTime>
 #include <QGuiApplication>
+#include <QLinearGradient>
 #include <QPainter>
 #include <QRegularExpression>
 #include <QStyle>
@@ -100,6 +101,18 @@ QColor CardDelegate::mutedChipColour(const QColor &chipColour)
     return QColor::fromHslF(h, s * kSaturationScale, l, a);
 }
 
+QRect CardDelegate::fadeRectFor(const QRect &card, const QRect &innermostSpine)
+{
+    // The EXCLUSIVE right edge, then a rect built from it: QRect::right() is
+    // inclusive, which is the trap CardLayout already documents.
+    const int end = card.left() + int(card.width() * kFadeFraction);
+    const int start = innermostSpine.isEmpty() ? card.left()
+                                               : innermostSpine.left();
+    if (end <= start)
+        return QRect();
+    return QRect(start, card.top(), end - start, card.height());
+}
+
 QColor CardDelegate::accentLineColour(const QColor &accountColour)
 {
     if (!accountColour.isValid())
@@ -178,6 +191,24 @@ void CardDelegate::paint(QPainter *painter, const QStyleOptionViewItem &option,
         root.data(ThreadListModel::AccountColourRole).value<QColor>();
     const QColor lineColour = accentLineColour(accountColour);
 
+    // The account's fade. Under everything but the chrome, so the selection
+    // highlight and the doomed-row tint still cover it: a selected row reading
+    // mostly as selection is expected, not a fault.
+    const QRect fade =
+        fadeRectFor(option.rect,
+                    card.spines.isEmpty() ? QRect() : card.spines.last());
+    if (!fade.isEmpty() && accountColour.isValid()) {
+        QColor from = lineColour;
+        // A reply's wash is weaker than its root's, so an expanded thread
+        // reads as one block with the root leading it.
+        from.setAlphaF(card.accentRect.isEmpty() ? 0.14 : 0.30);
+        QLinearGradient gradient(fade.topLeft(), fade.topRight());
+        gradient.setColorAt(0.0, from);
+        from.setAlphaF(0.0);
+        gradient.setColorAt(1.0, from);
+        painter->fillRect(fade, gradient);
+    }
+
     // The accent bar, thread cards only. Drawn after the chrome so the
     // selection highlight cannot cover it: which account a card belongs to
     // must stay readable on the row the user is looking at.
diff --git a/src/carddelegate.h b/src/carddelegate.h
index 1846359..5a0830c 100644
--- a/src/carddelegate.h
+++ b/src/carddelegate.h
@@ -77,6 +77,21 @@ public:
     /// Falls back to threadLineColour() for a thread with no account tag.
     static QColor accentLineColour(const QColor &accountColour);
 
+    /// Where the account's fade runs, given the card and the row's innermost
+    /// spine (an empty rect for a thread root, which has none).
+    ///
+    /// A root's fade starts at the card's left edge; a reply's starts at its
+    /// own spine, which IS its coloured left border, so the wash steps right
+    /// with the nesting. Both end at 60% of the card's width, so a deeper
+    /// reply's wash is shorter as well as further right.
+    ///
+    /// Static and rect-in, rect-out so the geometry is assertable without a
+    /// painter, for the same reason CardLayout is.
+    static QRect fadeRectFor(const QRect &card, const QRect &innermostSpine);
+
+    /// How far across the card the account's colour reaches.
+    static constexpr qreal kFadeFraction = 0.60;
+
     /// A tag chip's colour, drained for the SIBLING tier (item 111).
     ///
     /// Saturation only: hue stays so the tag is still recognisable, and