Started the Windows ProgressBar implemetnation. There's still an important detail left over...

basicctrls_windows.c

@@ -235,4 +235,7 @@ void setSpinboxEditSubclass(HWND hwnd, void *data)
 {
 	if ((*fv_SetWindowSubclass)(hwnd, spinboxEditSubProc, 0, (DWORD_PTR) data) == FALSE)
 		xpanic("error subclassing Spinbox edit control to give it its own event handler", GetLastError());
-}
+}
+
+// provided for cgo's benefit
+LPWSTR xPROGRESS_CLASS = PROGRESS_CLASS;

progressbar_windows.go

@@ -0,0 +1,52 @@
+// 4 november 2014
+
+package ui
+
+import (
+	"fmt"
+)
+
+// #include "winapi_windows.h"
+import "C"
+
+type progressbar struct {
+	*controlSingleHWND
+}
+
+func newProgressBar() ProgressBar {
+	hwnd := C.newControl(C.xPROGRESS_CLASS,
+		C.PBS_SMOOTH,
+		0)
+	p := &progressbar{
+		controlSingleHWND:		newControlSingleHWND(hwnd),
+	}
+	p.fpreferredSize = p.xpreferredSize
+	p.fnTabStops = func() int {
+		// progress bars are not tab stops
+		return 0
+	}
+	return p
+}
+
+func (p *progressbar) Percent() int {
+	return int(C.SendMessageW(p.hwnd, C.PBM_GETPOS, 0, 0))
+}
+
+func (p *progressbar) SetPercent(percent int) {
+	if percent < 0 || percent > 100 {
+		panic(fmt.Errorf("given ProgressBar percentage %d out of range", percent))
+	}
+	// TODO circumvent aero
+	C.SendMessageW(p.hwnd, C.PBM_SETPOS, C.WPARAM(percent), 0)
+}
+
+const (
+	// via http://msdn.microsoft.com/en-us/library/windows/desktop/dn742486.aspx
+	// this is the double-width option
+	progressbarWidth = 237
+	progressbarHeight = 8
+)
+
+func (p *progressbar) xpreferredSize(d *sizing) (width, height int) {
+	return fromdlgunitsX(progressbarWidth, d), fromdlgunitsY(progressbarHeight, d)
+}

winapi_windows.h

@@ -77,6 +77,7 @@ extern void textfieldHideInvalidBalloonTip(HWND);
 extern void setGroupSubclass(HWND, void *);
 extern HWND newUpDown(HWND, void *);
 extern void setSpinboxEditSubclass(HWND, void *);
+extern LPWSTR xPROGRESS_CLASS;
 
 // init_windows.c
 extern HINSTANCE hInstance;