Added uiStackRemove(), the first removal function in package ui! That took long enough... Now I can piece together all the control and parent logic...

stack.c

@@ -367,6 +367,22 @@ void uiStackAdd(uiControl *st, uiControl *c, int stretchy)
 		uiParentUpdate(s->parent);
 }
 
+void uiStackRemove(uiControl *st, uintptr_t index)
+{
+	stack *s = (stack *) (st->data);
+	uiControl *removed;
+	uintmax_t nAfter;
+
+	removed = s->controls[index].c;
+	nAfter = s->len - index - 1;
+	memmove(&(s->controls[index + 1]), &(s->controls[index]), nAfter * sizeof (stackControl));
+	s->len--;
+	if (s->parent != NULL) {
+		uiControlRemoveParent(removed);
+		uiParentUpdate(s->parent);
+	}
+}
+
 int uiStackPadded(uiControl *c)
 {
 	stack *s = (stack *) (c->data);

test.c

@@ -67,7 +67,7 @@ static void setCheckboxText(uiControl *b, void *data)
 }
 
 uiWindow *w;
-#define nStacks 9
+#define nStacks 11
 uiControl *stacks[nStacks];
 uiControl *spaced;
 
@@ -148,6 +148,22 @@ static void setLabelText(uiControl *b, void *data)
 	uiFreeText(text);
 }
 
+uiControl *firstStack;
+uiControl *secondStack;
+uiControl *movingLabel;
+
+static void moveToFirst(uiControl *c, void *data)
+{
+	uiStackRemove(secondStack, 1);
+	uiStackAdd(firstStack, movingLabel, 1);
+}
+
+static void moveToSecond(uiControl *c, void *data)
+{
+	uiStackRemove(firstStack, 1);
+	uiStackAdd(secondStack, movingLabel, 1);
+}
+
 int main(int argc, char *argv[])
 {
 	uiInitOptions o;
@@ -289,6 +305,24 @@ int main(int argc, char *argv[])
 	uiTabAddPage(tab, "Page 2", stacks[i]);
 	i++;
 
+	stacks[i] = uiNewHorizontalStack();
+	firstStack = stacks[i];
+	getButton = uiNewButton("Move Here");
+	uiButtonOnClicked(getButton, moveToFirst, NULL);
+	uiStackAdd(stacks[i], getButton, 0);
+	movingLabel = uiNewLabel("This label moves!");
+	uiStackAdd(stacks[i], movingLabel, 1);
+	uiStackAdd(stacks[page2stack], stacks[i], 0);;
+	i++;
+
+	stacks[i] = uiNewHorizontalStack();
+	secondStack = stacks[i];
+	getButton = uiNewButton("Move Here");
+	uiButtonOnClicked(getButton, moveToSecond, NULL);
+	uiStackAdd(stacks[i], getButton, 0);
+	uiStackAdd(stacks[page2stack], stacks[i], 0);
+	i++;
+
 	if (i != nStacks) {
 		fprintf(stderr, "forgot to update nStacks (expected %d)\n", i);
 		return 1;

ui.h

@@ -131,6 +131,7 @@ void uiButtonOnClicked(uiControl *, void (*)(uiControl *, void *), void *);
 uiControl *uiNewHorizontalStack(void);
 uiControl *uiNewVerticalStack(void);
 void uiStackAdd(uiControl *, uiControl *, int);
+void uiStackRemove(uiControl *, uintmax_t);
 int uiStackPadded(uiControl *);
 void uiStackSetPadded(uiControl *, int);