Installed the Z-order stuff into box.c; tab order is now correct. There are some loose ends, though.

redo/box.c

@@ -233,6 +233,18 @@ static void boxAppend(uiBox *ss, uiControl *c, int stretchy)
 {
 	struct box *b = (struct box *) ss;
 	struct boxControl *bc;
+	uintptr_t zorder;
+	int dozorder;
+	uintmax_t i;
+
+	// start the zorder with the *CURRENT* first child
+	// this is in case we're adding a new first child
+	dozorder = 0;
+	if (b->controls->len != 0) {
+		dozorder = 1;
+		bc = ptrArrayIndex(b->controls, struct boxControl *, 0);
+		zorder = uiControlStartZOrder(bc->c);
+	}
 
 	bc = uiNew(struct boxControl);
 	bc->c = c;
@@ -240,6 +252,13 @@ static void boxAppend(uiBox *ss, uiControl *c, int stretchy)
 	uiControlSetParent(bc->c, uiControl(b));
 	ptrArrayAppend(b->controls, bc);
 	uiControlQueueResize(uiControl(b));
+
+	// and now update the zorder for all controls
+	if (dozorder)
+		for (i = 0; i < b->controls->len; i++) {
+			bc = ptrArrayIndex(b->controls, struct boxControl *, i);
+			zorder = uiControlSetZOrder(bc->c, zorder);
+		}
 }
 
 static void boxDelete(uiBox *ss, uintmax_t index)

redo/windows/control.c

@@ -109,11 +109,19 @@ static void singleHWNDCommitDisable(uiControl *c)
 uintptr_t uiWindowsUtilStartZOrder(HWND hwnd)
 {
 	HWND insertAfter;
+	DWORD le;
 
 	// see http://stackoverflow.com/questions/30491418/
+	// also, the window at the beginning of the z-order has no previous window, so GetWindow() returns NULL
+	// we have to differentiate these error states
+	SetLastError(0);
 	insertAfter = GetWindow(hwnd, GW_HWNDPREV);
-	if (insertAfter == NULL)
-		logLastError("error getting insert after window in uiWindowsUtilStartZOrder()");
+	if (insertAfter == NULL) {
+		le = GetLastError();
+		SetLastError(le);		// just in case
+		if (le != 0)
+			logLastError("error getting insert after window in uiWindowsUtilStartZOrder()");
+	}
 	return (uintptr_t) insertAfter;
 }
 

redo/windows/tab.c

@@ -3,6 +3,7 @@
 
 // TODO
 // - dialog doesn't respond to WM_PRINTCLIENT in Windows Classic
+// - can't seem to tab away anymore
 
 struct tab {
 	uiTab t;