diff --git a/newcontrol_windows.c b/newcontrol_windows.c
index 1ff404b1..3a561c6e 100644
--- a/newcontrol_windows.c
+++ b/newcontrol_windows.c
@@ -1,37 +1,35 @@
 // 6 april 2015
 #include "uipriv_windows.h"
 
-typedef struct uiSingleHWNDControl uiSingleHWNDControl;
+typedef struct singleHWND singleHWND;
 
 struct uiSingleHWNDControl {
-	uiControl control;
 	HWND hwnd;
-	BOOL (*onWM_COMMAND)(uiControl *, WPARAM, LPARAM, void *, LRESULT *);
-	BOOL (*onWM_NOTIFY)(uiControl *, WPARAM, LPARAM, void *, LRESULT *);
-	void (*onWM_DESTROY)(uiControl *, void *);
-	void *onCommandNotifyDestroyData;
-	void (*preferredSize)(uiControl *, int, int, LONG, intmax_t *, intmax_t *);
-	void *data;
+	BOOL (*onWM_COMMAND)(uiControl *, WPARAM, LPARAM, LRESULT *);
+	BOOL (*onWM_NOTIFY)(uiControl *, WPARAM, LPARAM, LRESULT *);
+	void (*onWM_DESTROY)(uiControl *);
 	uintptr_t parent;
 };
 
-#define S(c) ((uiSingleHWNDControl *) (c))
-
 static void singleDestroy(uiControl *c)
 {
-	if (DestroyWindow(S(c)->hwnd) == 0)
+	singleHWND *s = (singleHWND *) (c->internal);
+
+	if (DestroyWindow(s->hwnd) == 0)
 		logLastError("error destroying control in singleDestroy()");
-	// the uiSingleHWNDControl is destroyed in the subclass procedure
+	// the data structures are destroyed in the subclass procedure
 }
 
 static uintptr_t singleHandle(uiControl *c)
 {
-	return (uintptr_t) (S(c)->hwnd);
+	singleHWND *s = (singleHWND *) (c->internal);
+
+	return (uintptr_t) (s->hwnd);
 }
 
 static void singleSetParent(uiControl *c, uintptr_t parent)
 {
-	uiSingleHWNDControl *s = S(c);
+	singleHWND *s = (singleHWND *) (c->internal);
 
 	s->parent = parent;
 	if (SetParent(s->hwnd, (HWND) (s->parent)) == NULL)
@@ -41,7 +39,7 @@ static void singleSetParent(uiControl *c, uintptr_t parent)
 
 static void singleRemoveParent(uiControl *c)
 {
-	uiSingleHWNDControl *s = S(c);
+	singleHWND *s = (singleHWND *) (c->internal);
 	uintptr_t oldparent;
 
 	oldparent = s->parent;
@@ -51,38 +49,32 @@ static void singleRemoveParent(uiControl *c)
 	updateParent(oldparent);
 }
 
-static uiSize singlePreferredSize(uiControl *c, uiSizing *d)
-{
-	uiSize size;
-
-	(*(S(c)->preferredSize))(c,
-		d->baseX, d->baseY, d->internalLeading,
-		&(size.width), &(size.height));
-	return size;
-}
-
 static void singleResize(uiControl *c, intmax_t x, intmax_t y, intmax_t width, intmax_t height, uiSizing *d)
 {
-	if (MoveWindow(S(c)->hwnd, x, y, width, height, TRUE) == 0)
+	singleHWND *s = (singleHWND *) (c->internal);
+
+	if (MoveWindow(s->hwnd, x, y, width, height, TRUE) == 0)
 		logLastError("error moving control in singleResize()");
 }
 
 static LRESULT CALLBACK singleSubclassProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam, UINT_PTR uIdSubclass, DWORD_PTR dwRefData)
 {
-	uiSingleHWNDControl *c = (uiSingleHWNDControl *) dwRefData;
+	uiControl *c = (uiControl *) dwRefData;
+	singleHWND *s = (singleHWND *) (c->internal);
 	LRESULT lResult;
 
 	switch (uMsg) {
 	case msgCOMMAND:
-		if ((*(c->onWM_COMMAND))((uiControl *) c, wParam, lParam, c->onCommandNotifyDestroyData, &lResult) != FALSE)
+		if ((*(s->onWM_COMMAND))(c, wParam, lParam, &lResult) != FALSE)
 			return lResult;
 		break;
 	case msgNOTIFY:
-		if ((*(c->onWM_NOTIFY))((uiControl *) c, wParam, lParam, c->onCommandNotifyDestroyData, &lResult) != FALSE)
+		if ((*(s->onWM_NOTIFY))(c, wParam, lParam, &lResult) != FALSE)
 			return lResult;
 		break;
 	case WM_DESTROY:
-		(*(c->onWM_DESTROY))((uiControl *) c, c->onCommandNotifyDestroyData);
+		(*(s->onWM_DESTROY))(c);
+		uiFree(s);
 		uiFree(c);
 		break;
 	case WM_NCDESTROY:
@@ -95,51 +87,44 @@ static LRESULT CALLBACK singleSubclassProc(HWND hwnd, UINT uMsg, WPARAM wParam,
 
 uiControl *uiWindowsNewControl(uiWindowsNewControlParams *p)
 {
-	uiSingleHWNDControl *c;
+	uiControl *c;
+	singleHWND *s;
 
-	c = uiNew(uiSingleHWNDControl);
-	c->hwnd = CreateWindowExW(p->dwExStyle,
+	s = uiNew(singleHWND);
+	s->hwnd = CreateWindowExW(p->dwExStyle,
 		p->lpClassName, p->lpWindowName,
 		p->dwStyle | WS_CHILD | WS_VISIBLE,
 		0, 0,
 		100, 100,
 		// TODO specify control IDs properly
 		initialParent, NULL, p->hInstance, NULL);
-	if (c->hwnd == NULL)
+	if (s->hwnd == NULL)
 		logLastError("error creating control in uiWindowsNewControl()");
+	s->onWM_COMMAND = p->onWM_COMMAND;
+	s->onWM_NOTIFY = p->onWM_NOTIFY;
+	s->onWM_DESTROY = p->onWM_DESTROY;
 
-	c->control.destroy = singleDestroy;
-	c->control.handle = singleHandle;
-	c->control.setParent = singleSetParent;
-	c->control.removeParent = singleRemoveParent;
-	c->control.preferredSize = singlePreferredSize;
-	c->control.resize = singleResize;
+	c = uiNew(uiControl);
+	c->internal = s;
+	c->destroy = singleDestroy;
+	c->handle = singleHandle;
+	c->setParent = singleSetParent;
+	c->removeParent = singleRemoveParent;
+	c->resize = singleResize;
 
-	c->onWM_COMMAND = p->onWM_COMMAND;
-	c->onWM_NOTIFY = p->onWM_NOTIFY;
-	c->onWM_DESTROY = p->onWM_DESTROY;
-	c->onCommandNotifyDestroyData = p->onCommandNotifyDestroyData;
-	c->preferredSize = p->preferredSize;
-
-	c->data = p->data;
-
-	if ((*fv_SetWindowSubclass)(c->hwnd, singleSubclassProc, 0, (DWORD_PTR) c) == FALSE)
+	if ((*fv_SetWindowSubclass)(s->hwnd, singleSubclassProc, 0, (DWORD_PTR) c) == FALSE)
 		logLastError("error subclassing Windows control in uiWindowsNewControl()");
 
-	return (uiControl *) c;
-}
-
-void *uiWindowsControlData(uiControl *c)
-{
-	return S(c)->data;
+	return c;
 }
 
 char *uiWindowsControlText(uiControl *c)
 {
+	singleHWND *s = (singleHWND *) (c->internal);
 	WCHAR *wtext;
 	char *text;
 
-	wtext = windowText(S(c)->hwnd);
+	wtext = windowText(s->hwnd);
 	text = toUTF8(wtext);
 	uiFree(wtext);
 	return text;
@@ -147,10 +132,11 @@ char *uiWindowsControlText(uiControl *c)
 
 void uiWindowsControlSetText(uiControl *c, const char *text)
 {
+	singleHWND *s = (singleHWND *) (c->internal);
 	WCHAR *wtext;
 
 	wtext = toUTF16(text);
-	if (SetWindowTextW(S(c)->hwnd, wtext) == 0)
+	if (SetWindowTextW(s->hwnd, wtext) == 0)
 		logLastError("error setting control text in uiWindowsControlSetText()");
 	uiFree(wtext);
 }
diff --git a/ui_windows.h b/ui_windows.h
index d1a95786..6932ff4f 100644
--- a/ui_windows.h
+++ b/ui_windows.h
@@ -8,6 +8,7 @@ This file assumes that you have included <windows.h> and "ui.h" beforehand. It p
 #define __UI_UI_WINDOWS_H__
 
 // uiWindowsNewControl() creates a new uiControl with the given Windows API control inside.
+// You will need to provide the preferredSize() method yourself.
 typedef struct uiWindowsNewControlParams uiWindowsNewControlParams;
 struct uiWindowsNewControlParams {
 	// These match the CreateWindowExW() function.
@@ -21,25 +22,24 @@ struct uiWindowsNewControlParams {
 	// ui redirects the message back and calls these functions.
 	// Store the result in *lResult and return any non-FALSE value (such as TRUE) to return the given result; return FALSE to pass the notification up to your window procedure.
 	// Note that these are only issued if they come from the uiControl itself; notifications from children of the uiControl (such as a header control) will be received normally.
-	BOOL (*onWM_COMMAND)(uiControl *c, WPARAM wParam, LPARAM lParam, void *data, LRESULT *lResult);
-	BOOL (*onWM_NOTIFY)(uiControl *c, WPARAM wParam, LPARAM lParam, void *data, LRESULT *lResult);
+	// TODO don't give WPARAM/LPARAM raw
+	BOOL (*onWM_COMMAND)(uiControl *c, WPARAM wParam, LPARAM lParam, LRESULT *lResult);
+	BOOL (*onWM_NOTIFY)(uiControl *c, WPARAM wParam, LPARAM lParam, LRESULT *lResult);
 	// This is called in WM_DESTROY.
-	void (*onWM_DESTROY)(uiControl *c, void *data);
-	// This is the data parameter to all three of the above.
-	void *onCommandNotifyDestroyData;
-
-	// This function is called when ui needs to know how to rearrange controls in a window.
-	// baseX and baseY are the base units used to convert between dialog units and pixels.
-	// internalLeading is the internal leading of the control font.
-	void (*preferredSize)(uiControl *c, int baseX, int baseY, LONG internalLeading, intmax_t *width, intmax_t *height);
-
-	// Data you can get with uiWindowsControlData()
-	void *data;
+	void (*onWM_DESTROY)(uiControl *c);
 };
 uiControl *uiWindowsNewControl(uiWindowsNewControlParams *);
-void *uiWindowsControlData(uiControl *);
 
-// use these in your preferredSize() implementation with baseX and baseY
+// This contains the Windows-specific parts of the uiSizing structure.
+// baseX and baseY are the dialog base units.
+// internalLeading is the standard control font's internal leading; labels in uiForms use this for correct Y positioning.
+struct uiSizingSys {
+	int baseX;
+	int baseY;
+	LONG internalLeading;
+};
+// Use these in your preferredSize() implementation with baseX and baseY.
+// TODO rename to DlgUnits?
 #define uiDlgUnitToX(dlg, baseX) MulDiv((dlg), baseX, 4)
 #define uiDlgUnitToY(dlg, baseY) MulDiv((dlg), baseY, 8)
 
diff --git a/uipriv_windows.h b/uipriv_windows.h
index 2c545eee..8e602882 100644
--- a/uipriv_windows.h
+++ b/uipriv_windows.h
@@ -54,12 +54,6 @@ extern WCHAR *windowText(HWND);
 
 // container_windows.c
 extern BOOL sharedWndProc(HWND, UINT, WPARAM, LPARAM, LRESULT *);
-struct uiSizing {
-	uiSizingCommon
-	int baseX;
-	int baseY;
-	LONG internalLeading;
-};
 extern void resize(uiControl *, HWND, RECT, RECT);
 
 // comctl32_windows.c