fix(gui): the Windows title bar answers as the caption, so Snap works again (#6825)

src/gui/MainWindow.cpp

@@ -3876,6 +3876,7 @@ static LRESULT frameHitCode(WindowChrome::FrameHit hit)
     case WindowChrome::FrameHit::TopRight:    return HTTOPRIGHT;
     case WindowChrome::FrameHit::BottomLeft:  return HTBOTTOMLEFT;
     case WindowChrome::FrameHit::BottomRight: return HTBOTTOMRIGHT;
+    case WindowChrome::FrameHit::Caption:     return HTCAPTION;
     case WindowChrome::FrameHit::Client:      break;
     }
     return HTCLIENT;
@@ -3887,7 +3888,8 @@ bool MainWindow::nativeEvent(const QByteArray& eventType, void* message, qintptr
     // Under the expanded client area Qt 6.12 answers WM_NCHITTEST by turning
     // the live mouse-button state into synthetic presses, which doubles real
     // clicks and closes menus as they open (#6272). Answer it here instead:
-    // the resize band at the edges, client area everywhere else.
+    // the resize band at the edges, the title bar's own background as the
+    // caption so Windows offers Snap (#6825), client area everywhere else.
     if (msg && result && msg->message == WM_NCHITTEST
         && windowFlags().testFlag(Qt::ExpandedClientAreaHint)) {
         RECT window{};
@@ -3903,7 +3905,18 @@ bool MainWindow::nativeEvent(const QByteArray& eventType, void* message, qintptr
         const QPoint point(GET_X_LPARAM(msg->lParam), GET_Y_LPARAM(msg->lParam));
         const QRect rect(window.left, window.top,
                          window.right - window.left, window.bottom - window.top);
-        *result = frameHitCode(WindowChrome::expandedFrameHit(point, rect, borderX, borderY));
+        // The bar's height is in logical pixels and WM_NCHITTEST is physical.
+        // Only the bar's empty background may answer HTCAPTION: over its tabs,
+        // menus and buttons that is what closed menus as they opened (#6272),
+        // and isSystemMoveAreaAt() is the same test the bar's own drag uses, so
+        // a point either starts a system move or stays client — never both.
+        int captionHeight = 0;
+        if (m_titleBar && m_titleBar->isVisible()
+            && m_titleBar->isSystemMoveAreaAt(QCursor::pos())) {
+            captionHeight = int(m_titleBar->height() * devicePixelRatioF());
+        }
+        *result = frameHitCode(
+            WindowChrome::expandedFrameHit(point, rect, borderX, borderY, captionHeight));
         return true;
     }
     // The whole window is client area, inset only while maximized so nothing

src/gui/WindowChrome.h

@@ -90,27 +90,47 @@ inline bool claimsWholeWindowAsClient(Qt::WindowFlags flags, const QOperatingSys
 // live mouse-button state into synthetic presses, doubling real clicks (#6272).
 // Physical pixels; `borderX` / `borderY` are the resize bands on the left/right
 // and top/bottom edges (0 while maximized).
-enum class FrameHit { Client, Left, Right, Top, Bottom, TopLeft, TopRight, BottomLeft, BottomRight };
-
-inline FrameHit expandedFrameHit(const QPoint& point, const QRect& window, int borderX, int borderY)
+//
+// `Caption` is the title bar's own draggable background: the answer Windows
+// needs before it will offer drag-to-edge Snap and list the window in the Snap
+// Assist picker (#6825). Only the caller knows which points are the bar's empty
+// background rather than its tabs, menus and buttons, so this reports Caption
+// for the whole `captionHeight` strip and MainWindow::nativeEvent narrows it
+// with TitleBar::isSystemMoveAreaAt() — answering HTCAPTION over an interactive
+// widget is what made menus close as they opened (#6272). A `captionHeight` of
+// 0 disables it, which is how every non-main window keeps the old behaviour.
+enum class FrameHit { Client, Caption, Left, Right, Top, Bottom, TopLeft, TopRight, BottomLeft, BottomRight };
+
+inline FrameHit expandedFrameHit(const QPoint& point, const QRect& window, int borderX, int borderY,
+                                 int captionHeight = 0)
 {
-    if (borderX <= 0 || borderY <= 0 || !window.contains(point)) {
+    if (!window.contains(point)) {
         return FrameHit::Client;
     }
-    const bool left = point.x() < window.left() + borderX;
-    const bool right = point.x() > window.right() - borderX;
-    const bool top = point.y() < window.top() + borderY;
-    const bool bottom = point.y() > window.bottom() - borderY;
-    if (left) {
-        return top ? FrameHit::TopLeft : bottom ? FrameHit::BottomLeft : FrameHit::Left;
-    }
-    if (right) {
-        return top ? FrameHit::TopRight : bottom ? FrameHit::BottomRight : FrameHit::Right;
+    // Maximized and fullscreen have no resize band, but the caption still has
+    // to answer or the window cannot be dragged back off the top of the screen.
+    if (borderX > 0 && borderY > 0) {
+        const bool left = point.x() < window.left() + borderX;
+        const bool right = point.x() > window.right() - borderX;
+        const bool top = point.y() < window.top() + borderY;
+        const bool bottom = point.y() > window.bottom() - borderY;
+        if (left) {
+            return top ? FrameHit::TopLeft : bottom ? FrameHit::BottomLeft : FrameHit::Left;
+        }
+        if (right) {
+            return top ? FrameHit::TopRight : bottom ? FrameHit::BottomRight : FrameHit::Right;
+        }
+        if (top) {
+            return FrameHit::Top;
+        }
+        if (bottom) {
+            return FrameHit::Bottom;
+        }
     }
-    if (top) {
-        return FrameHit::Top;
+    if (captionHeight > 0 && point.y() < window.top() + captionHeight) {
+        return FrameHit::Caption;
     }
-    return bottom ? FrameHit::Bottom : FrameHit::Client;
+    return FrameHit::Client;
 }
 
 // Windows: the native window rect that gives `client`, keeping the frame the

tests/unified_title_bar_test.cpp

@@ -204,6 +204,56 @@ int main(int argc, char** argv)
               "the left/right band and the top/bottom band follow their own widths");
     }
 
+    // ── The title bar answers as the caption so Windows offers Snap (#6825) ──
+    {
+        using Hit = WindowChrome::FrameHit;
+        const QRect win(100, 50, 1000, 600);
+        const int band = 8;
+        const int caption = 52;
+
+        // Without a caption height nothing changes: every existing caller
+        // (and every window that is not the main window) keeps the old answer.
+        check(WindowChrome::expandedFrameHit(QPoint(600, 70), win, band, band) == Hit::Client,
+              "the title bar is client area when no caption height is given");
+
+        // The bar's background drags the window, which is what Windows needs
+        // before it offers drag-to-edge Snap and the Snap Assist picker.
+        check(WindowChrome::expandedFrameHit(QPoint(600, 70), win, band, band, caption)
+                  == Hit::Caption,
+              "the title bar's background is the caption");
+        check(WindowChrome::expandedFrameHit(QPoint(600, 101), win, band, band, caption)
+                  == Hit::Caption,
+              "the last pixel of the caption strip still drags");
+        check(WindowChrome::expandedFrameHit(QPoint(600, 102), win, band, band, caption)
+                  == Hit::Client,
+              "the first pixel below the caption is client area");
+
+        // The resize band wins over the caption, or the top edge and the top
+        // corners would stop resizing the moment the caption was introduced.
+        check(WindowChrome::expandedFrameHit(QPoint(600, 50), win, band, band, caption) == Hit::Top,
+              "the top resize band still wins over the caption");
+        check(WindowChrome::expandedFrameHit(QPoint(101, 51), win, band, band, caption)
+                  == Hit::TopLeft
+                  && WindowChrome::expandedFrameHit(QPoint(1098, 51), win, band, band, caption)
+                      == Hit::TopRight,
+              "the top corners still resize under the caption");
+        check(WindowChrome::expandedFrameHit(QPoint(101, 70), win, band, band, caption)
+                  == Hit::Left,
+              "the side band still resizes alongside the caption");
+
+        // Maximized: no resize band, but the caption must still answer or the
+        // window cannot be dragged back down off the top of the screen.
+        check(WindowChrome::expandedFrameHit(QPoint(600, 70), win, 0, 0, caption) == Hit::Caption,
+              "the caption answers while maximized, so the window can be dragged free");
+        check(WindowChrome::expandedFrameHit(QPoint(600, 350), win, 0, 0, caption) == Hit::Client,
+              "the body is still client area while maximized");
+
+        // A point outside the window is never the caption.
+        check(WindowChrome::expandedFrameHit(QPoint(20, 20), win, band, band, caption)
+                  == Hit::Client,
+              "a point outside the window is not the caption");
+    }
+
     // ── Native client-rect save/restore for the Windows expanded frame (#6303) ──
     {
         // Rects measured on Windows 10: the expanded frame keeps an 8 px resize