app/internal/window: don't second guess UI scale

Before this change, Gio tries hard to come up with a reasonable UI scale
factor on desktop OSes derived from the physical dimensions and
resolution of connected monitors. Gio also attempts to detect the user
specified system UI scale and apply it.

However, all that is complex and misguided:

- The UI scale should not depend on whatever monitor is connected at
program startup - For multiple monitors, it's unclear which one to base
the scale off.  - Applying both a monitor derived scale *and* the user
specified scale is wrong, because the user scale is relative to some
fixed scale, not Gio's derived scale.  - With an automatic scale, Gio
does not respect user preference and will not have a similar scale to
other programs on the desktop.

Get rid of the the automatic UI scale detection and rely only on the
user scale.

Updates gio#53

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/internal/window/os_windows.go

@@ -79,7 +79,9 @@ const (
 
 	_INFINITE = 0xFFFFFFFF
 
-	_LOGPIXELSX = 88
+	_MDT_EFFECTIVE_DPI = 0
+
+	_MONITOR_DEFAULTTOPRIMARY = 1
 
 	_SIZE_MAXIMIZED = 2
 	_SIZE_MINIMIZED = 1
@@ -110,6 +112,7 @@ const (
 	_WM_CANCELMODE  = 0x001F
 	_WM_CHAR        = 0x0102
 	_WM_CREATE      = 0x0001
+	_WM_DPICHANGED  = 0x02E0
 	_WM_DESTROY     = 0x0002
 	_WM_KEYDOWN     = 0x0100
 	_WM_KEYUP       = 0x0101
@@ -270,6 +273,9 @@ func windowProc(hwnd syscall.Handle, msg uint32, wParam, lParam uintptr) uintptr
 		}
 		// The message is processed.
 		return 1
+	case _WM_DPICHANGED:
+		// Let Windows know we're prepared for runtime DPI changes.
+		return 1
 	case _WM_KEYDOWN, _WM_SYSKEYDOWN:
 		if n, ok := convertKeyCode(wParam); ok {
 			cmd := key.Event{Name: n}
@@ -478,12 +484,9 @@ func convertKeyCode(code uintptr) (rune, bool) {
 }
 
 func configForDC(hdc syscall.Handle) config {
-	dpi := getDeviceCaps(hdc, _LOGPIXELSX)
-	ppdp := float32(dpi) * inchPrDp * monitorScale
-	// Force a minimum density to keep text legible and to handle bogus output geometry.
-	if ppdp < minDensity {
-		ppdp = minDensity
-	}
+	hmon := monitorFromPoint(point{}, _MONITOR_DEFAULTTOPRIMARY)
+	dpi := getDpiForMonitor(hmon, _MDT_EFFECTIVE_DPI)
+	ppdp := float32(dpi) * inchPrDp
 	return config{
 		pxPerDp: ppdp,
 		pxPerSp: ppdp,
@@ -508,6 +511,7 @@ var (
 	_GetMessageTime              = user32.NewProc("GetMessageTime")
 	_KillTimer                   = user32.NewProc("KillTimer")
 	_LoadCursor                  = user32.NewProc("LoadCursorW")
+	_MonitorFromPoint            = user32.NewProc("MonitorFromPoint")
 	_MsgWaitForMultipleObjectsEx = user32.NewProc("MsgWaitForMultipleObjectsEx")
 	_PeekMessage                 = user32.NewProc("PeekMessageW")
 	_PostMessage                 = user32.NewProc("PostMessageW")
@@ -526,8 +530,8 @@ var (
 	_UnregisterClass             = user32.NewProc("UnregisterClassW")
 	_UpdateWindow                = user32.NewProc("UpdateWindow")
 
-	gdi32          = syscall.NewLazySystemDLL("gdi32")
-	_GetDeviceCaps = gdi32.NewProc("GetDeviceCaps")
+	shcore            = syscall.NewLazySystemDLL("shcore")
+	_GetDpiForMonitor = shcore.NewProc("GetDpiForMonitor")
 )
 
 func getModuleHandle() (syscall.Handle, error) {
@@ -596,9 +600,10 @@ func getDC(hwnd syscall.Handle) (syscall.Handle, error) {
 	return syscall.Handle(hdc), nil
 }
 
-func getDeviceCaps(hdc syscall.Handle, index int32) int {
-	c, _, _ := _GetDeviceCaps.Call(uintptr(hdc), uintptr(index))
-	return int(c)
+func getDpiForMonitor(hmonitor syscall.Handle, dpiType uint32) int {
+	var dpiX, dpiY uintptr
+	_GetDpiForMonitor.Call(uintptr(hmonitor), uintptr(dpiType), uintptr(unsafe.Pointer(&dpiX)), uintptr(unsafe.Pointer(&dpiY)))
+	return int(dpiX)
 }
 
 func getKeyState(nVirtKey int32) int16 {
@@ -636,6 +641,11 @@ func loadCursor(curID uint16) (syscall.Handle, error) {
 	return syscall.Handle(h), nil
 }
 
+func monitorFromPoint(pt point, flags uint32) syscall.Handle {
+	r, _, _ := _MonitorFromPoint.Call(uintptr(pt.x), uintptr(pt.y), uintptr(flags))
+	return syscall.Handle(r)
+}
+
 func msgWaitForMultipleObjectsEx(nCount uint32, pHandles uintptr, millis, mask, flags uint32) (uint32, error) {
 	r, _, err := _MsgWaitForMultipleObjectsEx.Call(uintptr(nCount), pHandles, uintptr(millis), uintptr(mask), uintptr(flags))
 	res := uint32(r)