Center windows on their transient parent where possible

This is only really relevant in practice on MacOS and Windows.  On X11, the
window manager places new windows where it pleases.

3 files changed, 56 insertions, 16 deletions

diff --git a/src/mac.m b/src/mac.m
index 841a154..77c825f 100644
--- a/src/mac.m
+++ b/src/mac.m
@@ -91,7 +91,7 @@ utiForMimeType(const NSString* const mimeType)
 }
 
 static NSRect
-rectToScreen(NSScreen* screen, NSRect rect)
+rectToScreen(const NSScreen* screen, NSRect rect)
 {
   const double screenHeight = [screen frame].size.height;
 
@@ -99,6 +99,17 @@ rectToScreen(NSScreen* screen, NSRect rect)
   return rect;
 }
 
+static NSRect
+rectFromScreen(const NSScreen* screen, NSRect rect)
+{
+  /* The math happens to work out to the same expression because this is just
+     an inversion of the Y axis based on the screen height.  This function is
+     preserved to make the code clearer and more future-proof since the use
+     cases are distinctly either a "from" or "to" conversion. */
+
+  return rectToScreen(screen, rect);
+}
+
 static NSScreen*
 viewScreen(const PuglView* view)
 {
@@ -1109,6 +1120,25 @@ updateSizeHints(PuglView* const view)
   }
 }
 
+static void
+puglMacSetDefaultPosition(PuglView* const view)
+{
+  // Get a bounding rect from the transient parent or the screen
+  const NSScreen* const screen = viewScreen(view);
+  const NSRect          boundsPt =
+    rectFromScreen(screen,
+                   view->transientParent
+                     ? [[(const NSView*)view->transientParent window] frame]
+                     : [screen frame]);
+
+  // Center the frame around the center of the bounding rectangle
+  const NSRect boundsPx = nsRectFromPoints(view, boundsPt);
+  const double centerX  = boundsPx.origin.x + boundsPx.size.width / 2;
+  const double centerY  = boundsPx.origin.y + boundsPx.size.height / 2;
+  view->frame.x         = (PuglCoord)(centerX - (view->frame.width / 2U));
+  view->frame.y         = (PuglCoord)(centerY - (view->frame.height / 2U));
+}
+
 PuglStatus
 puglRealize(PuglView* view)
 {
@@ -1156,11 +1186,7 @@ puglRealize(PuglView* view)
 
   // Center top-level windows if a position has not been set
   if (!view->parent && !view->frame.x && !view->frame.y) {
-    const double screenWidthPx  = [screen frame].size.width * scaleFactor;
-    const double screenHeightPx = [screen frame].size.height * scaleFactor;
-
-    view->frame.x = (PuglCoord)((screenWidthPx - view->frame.width) / 2.0);
-    view->frame.y = (PuglCoord)((screenHeightPx - view->frame.height) / 2.0);
+    puglMacSetDefaultPosition(view);
   }
 
   const NSRect framePx = rectToNsRect(view->frame);
diff --git a/src/types.h b/src/types.h
index 785eae8..da0a295 100644
--- a/src/types.h
+++ b/src/types.h
@@ -21,6 +21,12 @@ typedef struct PuglInternalsImpl PuglInternals;
 /// View hints
 typedef int PuglHints[PUGL_NUM_VIEW_HINTS];
 
+/// View position (both X and Y coordinates) or general point
+typedef struct {
+  PuglCoord x;
+  PuglCoord y;
+} PuglPoint;
+
 /// View size (both X and Y coordinates)
 typedef struct {
   PuglSpan width;
diff --git a/src/win.c b/src/win.c
index d3bc4fd..4c4c815 100644
--- a/src/win.c
+++ b/src/win.c
@@ -1502,6 +1502,21 @@ puglWinGetPixelFormatDescriptor(const PuglHints hints)
   return pfd;
 }
 
+static void
+puglWinSetDefaultPosition(PuglView* const view)
+{
+  // Get a bounding rect from the "nearest" parent or parent-like window
+  const HWND hwnd = puglWinGetWindow(view);
+  RECT       rect = {0, 0, 0, 0};
+  GetWindowRect(hwnd ? hwnd : GetDesktopWindow(), &rect);
+
+  // Center the frame around the center of the bounding rectangle
+  const LONG centerX = rect.left + (rect.right - rect.left) / 2;
+  const LONG centerY = rect.top + (rect.bottom - rect.top) / 2;
+  view->frame.x      = (PuglCoord)(centerX - (view->frame.width / 2U));
+  view->frame.y      = (PuglCoord)(centerY - (view->frame.height / 2U));
+}
+
 PuglStatus
 puglWinCreateWindow(PuglView* const   view,
                     const char* const title,
@@ -1514,14 +1529,7 @@ puglWinCreateWindow(PuglView* const   view,
 
   // Center top-level windows if a position has not been set
   if (!view->parent && !view->frame.x && !view->frame.y) {
-    RECT desktopRect;
-    GetClientRect(GetDesktopWindow(), &desktopRect);
-
-    const int screenWidth  = desktopRect.right - desktopRect.left;
-    const int screenHeight = desktopRect.bottom - desktopRect.top;
-
-    view->frame.x = (PuglCoord)((screenWidth - view->frame.width) / 2);
-    view->frame.y = (PuglCoord)((screenHeight - view->frame.height) / 2);
+    puglWinSetDefaultPosition(view);
   }
 
   // The meaning of "parent" depends on the window type (WS_CHILD)
@@ -1539,8 +1547,8 @@ puglWinCreateWindow(PuglView* const   view,
                                className,
                                title,
                                winFlags,
-                               CW_USEDEFAULT,
-                               CW_USEDEFAULT,
+                               wr.left,
+                               wr.right,
                                wr.right - wr.left,
                                wr.bottom - wr.top,
                                (HWND)parent,