put the modal up in the same frame as the dim behind it

The layer was shown and painted before the modal it hosts was placed, so
opening one -- the layout dialog from the shipyard panel, say -- painted a
frame of bare dim over the world before the dialog appeared, which reads
as a black flash. The modal is now placed and shown while the layer is
still hidden, and showing the layer paints both at once.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ne3mejABZoLWKLh8fgpM3x

src/ui/ModalLayer.cpp

@@ -53,11 +53,18 @@ int ModalLayer::execute(ModalDialog& content, const QRect& anchorRect)
                                                 : m_stack.back().host->geometry());
 
     m_stack.push_back(HostedModal{ &content, host });
-    updateVisibility();
+
+    // Placed and shown before the layer itself is, so that the frame the layer first
+    // paints already carries the modal. Showing the layer first put the dim on screen a
+    // frame before anything stood on it, which read as a black flash
+    // (REQ-UI-MODAL-DIM). Placing works while the layer is hidden: its geometry is the
+    // window's whether or not it is shown, and a child shown under a hidden parent
+    // appears with it.
     raise();
     place(*host, content, anchor);
     host->show();
     content.show();
+    updateVisibility();
 
     // The modal's own focus widget where it has one -- a name dialog puts the caret in
     // its line edit -- and the modal itself otherwise, so keys reach it and not the game
@@ -209,8 +216,9 @@ void ModalLayer::updateVisibility()
     {
         raise();
         show();
-        // Force an immediate synchronous paint so the dim is up before whatever the
+        // Painted at once rather than at the next paint event, so no undimmed frame of
-        // caller does next; no undimmed frame flashes through.
+        // the world flashes through in between. The modal standing on the layer is
+        // already placed and shown by now, so this one paint puts up both (execute()).
         repaint();
     }
     else