macOS: Don't make frameless tool windows miniaturizable

Amends 556fbbea6e, which breaks
translucency of frameless tool windows. Possibly a bug in macOS, a
window being made miniaturizable should not have side effects on
background or translucency.

Pick-to: 6.2 6.1 5.15
Fixes: QTBUG-95042
Task-number: QTBUG-65637
Change-Id: I651196e3cf8a9c59b250e5f1b4064cc4ded02372
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/cocoa/qcocoawindow.mm

@@ -525,8 +525,10 @@ NSUInteger QCocoaWindow::windowStyleMask(Qt::WindowFlags flags)
     if (frameless) {
         // Frameless windows do not display the traffic lights buttons for
         // e.g. minimize, however StyleMaskMiniaturizable is required to allow
-        // programatic minimize.
-        styleMask |= NSWindowStyleMaskMiniaturizable;
+        // programatic minimize. However, for framless tool windows (e.g. dock windows)
+        // we don't want that, as it breaks translucency.
+        if (type != Qt::Tool)
+            styleMask |= NSWindowStyleMaskMiniaturizable;
     } else if (flags & Qt::CustomizeWindowHint) {
         if (flags & Qt::WindowTitleHint)
             styleMask |= NSWindowStyleMaskTitled;