Win32 Scrollbar clips text - c++

Question

When I resize my window and then use the scollbar, the text gets clipped. When I resize the window again, the clipped text shows properly again.
Why does this happen, and how can I prevent it?
Screenshot:
http://i.imgur.com/CLHgowx.png
main.cpp:
#include <Windows.h>
#include "sysmets.h"
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE prevInstance, PSTR szCmdLine, int iCmdShow)
{
static TCHAR szAppName[] = TEXT("Scratch");
HWND hwnd;
MSG msg;
WNDCLASS wndclass;
wndclass.style = CS_HREDRAW | CS_VREDRAW;
wndclass.lpfnWndProc = WndProc;
wndclass.cbClsExtra = 0;
wndclass.cbWndExtra = 0;
wndclass.hInstance = hInstance;
wndclass.hIcon = LoadIcon(NULL, IDI_APPLICATION);
wndclass.hCursor = LoadCursor(NULL, IDC_ARROW);
wndclass.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH);
wndclass.lpszMenuName = NULL;
wndclass.lpszClassName = szAppName;
if(!RegisterClass(&wndclass))
{
MessageBox(NULL, TEXT("Program requires Windows NT!"), szAppName, MB_ICONERROR);
return 0;
}
hwnd = CreateWindow(szAppName, TEXT("Scratch"),
WS_OVERLAPPEDWINDOW | WS_VSCROLL,
CW_USEDEFAULT, CW_USEDEFAULT,
CW_USEDEFAULT, CW_USEDEFAULT,
NULL, NULL, hInstance, NULL);
ShowWindow(hwnd, iCmdShow);
UpdateWindow(hwnd);
while(GetMessage(&msg, NULL, 0, 0))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
return msg.wParam;
}
LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
HDC hdc;
TEXTMETRIC tm;
PAINTSTRUCT ps;
SCROLLINFO si;
static int cxChar, cyChar, cxClient, cyClient, cxCaps;
int i, y, iPaintBeg, iPaintEnd, iVertPos;
TCHAR buffer[10];
switch(message)
{
case WM_CREATE:
hdc = GetDC(hwnd);
GetTextMetrics(hdc, &tm);
cxChar = tm.tmAveCharWidth;
cyChar = tm.tmHeight + tm.tmExternalLeading;
cxCaps = (tm.tmPitchAndFamily & 1 ? 3 : 2) * cxChar / 2;
ReleaseDC(hwnd, hdc);
return 0;
case WM_SIZE:
cxClient = LOWORD(lParam);
cyClient = HIWORD(lParam);
si.cbSize = sizeof(SCROLLINFO);
si.fMask = SIF_PAGE | SIF_RANGE;
si.nMin = 0;
si.nMax = NUMLINES - 1;
si.nPage = cyClient / cyChar;
SetScrollInfo(hwnd, SB_VERT, &si, TRUE);
return 0;
case WM_PAINT:
hdc = BeginPaint(hwnd, &ps);
si.cbSize = sizeof(SCROLLINFO);
si.fMask = SIF_POS;
GetScrollInfo(hwnd, SB_VERT, &si);
iVertPos = si.nPos;
iPaintBeg = max(0, iVertPos + ps.rcPaint.top / cyChar);
iPaintEnd = min(NUMLINES - 1, iVertPos + ps.rcPaint.bottom / cyChar);
for (i = iPaintBeg; i < iPaintEnd; i++)
{
y = cyChar * (i - iVertPos);
TextOut(hdc, 0, y, sysmetrics[i].szLabel, lstrlen(sysmetrics[i].szLabel));
TextOut(hdc, 22 * cxCaps, y, sysmetrics[i].szDesc, lstrlen(sysmetrics[i].szDesc));
SetTextAlign(hdc, TA_TOP | TA_RIGHT);
TextOut(hdc, 60 * cxCaps, y, buffer, wsprintf(buffer, TEXT("%5d") ,GetSystemMetrics(sysmetrics[i].iIndex)));
SetTextAlign(hdc, TA_TOP | TA_LEFT);
}
EndPaint(hwnd, &ps);
return 0;
case WM_VSCROLL:
si.cbSize = sizeof(SCROLLINFO);
si.fMask = SIF_ALL;
GetScrollInfo(hwnd, SB_VERT, &si);
iVertPos = si.nPos;
switch (LOWORD(wParam))
{
case SB_LINEUP:
si.nPos -= 1;
break;
case SB_LINEDOWN:
si.nPos += 1;
break;
case SB_PAGEUP:
si.nPos -= si.nPage;
break;
case SB_PAGEDOWN:
si.nPos += si.nPage;
break;
case SB_THUMBTRACK:
si.nPos = si.nTrackPos;
break;
default:
break;
}
SetScrollInfo(hwnd, SB_VERT, &si, TRUE);
GetScrollInfo(hwnd, SB_VERT, &si);
if(iVertPos != si.nPos)
{
ScrollWindow(hwnd, 0, cyChar * (iVertPos - si.nPos), NULL, NULL);
}
UpdateWindow(hwnd);
return 0;
case WM_DESTROY:
PostQuitMessage(0);
return 0;
}
return DefWindowProc(hwnd, message, wParam, lParam);
}
sysmets.h (defines NUMLINES and sysmetrics struct)
#ifndef SYSMETS_H_
#define SYSMETS_H_
#include <windows.h>
#define NUMLINES ((int) sizeof(sysmetrics) / sizeof(sysmetrics[0]))
struct
{
int iIndex;
TCHAR *szLabel;
TCHAR *szDesc;
}
sysmetrics [] =
{
SM_CXSCREEN, TEXT("SM_CXSCREEN"), TEXT("Screen Width in pixels."),
SM_CYSCREEN, TEXT("SM_CYSCREEN"), TEXT("Screen Height in pixels."),
SM_CXVSCROLL, TEXT("SM_CXVSCROLL"), TEXT("Vertical Scroll Width"),
SM_CYHSCROLL, TEXT("SM_CYHSCROLL"), TEXT("Horizontal Scroll Height"),
SM_CYCAPTION, TEXT("SM_CYCAPTION"), TEXT("Caption Bar Height"),
SM_CXBORDER, TEXT("SM_CXBORDER"), TEXT("Window Border Width"),
SM_CYBORDER, TEXT("SM_CYBORDER"), TEXT("Window Border Height"),
SM_CXFIXEDFRAME, TEXT("SM_CXFIXEDFRAME"), TEXT("Dialog Window Frame Width"),
SM_CYFIXEDFRAME, TEXT("SM_CYFIXEDFRAME"), TEXT("Dialog Window Frame Height"),
SM_CYVTHUMB, TEXT("SM_CYVTHUMB"), TEXT("Vertical scroll thumb height"),
SM_CXHTHUMB, TEXT("SM_CXHTHUMB"), TEXT("Horizontal scroll thumb width"),
SM_CXICON, TEXT("SM_CXICON"), TEXT("Icon width"),
SM_CYICON, TEXT("SM_CYICON"), TEXT("Icon height"),
SM_CXCURSOR, TEXT("SM_CXCURSOR"), TEXT("Cursor width"),
SM_CYCURSOR, TEXT("SM_CYCURSOR"),TEXT("Cursor height"),
SM_CYMENU, TEXT("SM_CYMENU"), TEXT("Menu bar height"),
SM_CXFULLSCREEN, TEXT("SM_CXFULLSCREEN"), TEXT("Full screen client area width"),
SM_CYFULLSCREEN, TEXT("SM_CYFULLSCREEN"), TEXT("Full screen client area height"),
SM_CYKANJIWINDOW, TEXT("SM_CYKANJIWINDOW"), TEXT("Kanji window height"),
SM_MOUSEPRESENT, TEXT("SM_MOUSEPRESENT"), TEXT("Mouse present flag"),
SM_CYVSCROLL, TEXT("SM_CYVSCROLL"), TEXT("Vertical scroll arrow height"),
SM_CXHSCROLL, TEXT("SM_CXHSCROLL"), TEXT("Horizontal scroll arrow width"),
SM_DEBUG, TEXT("SM_DEBUG"), TEXT("Debug version flag"),
SM_SWAPBUTTON, TEXT("SM_SWAPBUTTON"), TEXT("Mouse buttons swapped flag"),
SM_CXMIN, TEXT("SM_CXMIN"), TEXT("Minimum window width"),
SM_CYMIN, TEXT("SM_CYMIN"), TEXT("Minimum window height"),
SM_CXSIZE, TEXT("SM_CXSIZE"), TEXT("Min/Max/Close button width"),
SM_CYSIZE, TEXT("SM_CYSIZE"), TEXT("Min/Max/Close button height"),
SM_CXSIZEFRAME, TEXT("SM_CXSIZEFRAME"), TEXT("Window sizing frame width"),
SM_CYSIZEFRAME, TEXT("SM_CYSIZEFRAME"), TEXT("Window sizing frame height"),
SM_CXMINTRACK, TEXT("SM_CXMINTRACK"), TEXT("Minimum window tracking width"),
SM_CYMINTRACK, TEXT("SM_CYMINTRACK"), TEXT("Minimum window tracking height"),
SM_CXDOUBLECLK, TEXT("SM_CXDOUBLECLK"), TEXT("Double click x tolerance"),
SM_CYDOUBLECLK, TEXT("SM_CYDOUBLECLK"), TEXT("Double click y tolerance"),
SM_CXICONSPACING, TEXT("SM_CXICONSPACING"), TEXT("Horizontal icon spacing"),
SM_CYICONSPACING, TEXT("SM_CYICONSPACING"), TEXT("Vertical icon spacing"),
SM_MENUDROPALIGNMENT, TEXT("SM_MENUDROPALIGNMENT"), TEXT("Left or right menu drop"),
SM_PENWINDOWS, TEXT("SM_PENWINDOWS"), TEXT("Pen extensions installed"),
SM_DBCSENABLED, TEXT("SM_DBCSENABLED"), TEXT("Double-Byte Char Set enabled"),
SM_CMOUSEBUTTONS, TEXT("SM_CMOUSEBUTTONS"), TEXT("Number of mouse buttons"),
SM_SECURE, TEXT("SM_SECURE"), TEXT("Security present flag"),
SM_CXEDGE, TEXT("SM_CXEDGE"), TEXT("3-D border width"),
SM_CYEDGE, TEXT("SM_CYEDGE"), TEXT("3-D border height"),
SM_CXMINSPACING, TEXT("SM_CXMINSPACING"), TEXT("Minimized window spacing width"),
SM_CYMINSPACING, TEXT("SM_CYMINSPACING"), TEXT("Minimized window spacing height"),
SM_CXSMICON, TEXT("SM_CXSMICON"), TEXT("Small icon width"),
SM_CYSMICON, TEXT("SM_CYSMICON"), TEXT("Small icon height"),
SM_CYSMCAPTION, TEXT("SM_CYSMCAPTION"), TEXT("Small caption height"),
SM_CXSMSIZE, TEXT("SM_CXSMSIZE"), TEXT("Small caption button width"),
SM_CYSMSIZE, TEXT("SM_CYSMSIZE"), TEXT("Small caption button height"),
SM_CXMENUSIZE, TEXT("SM_CXMENUSIZE"), TEXT("Menu bar button width"),
SM_CYMENUSIZE, TEXT("SM_CYMENUSIZE"), TEXT("Menu bar button height"),
SM_ARRANGE, TEXT("SM_ARRANGE"), TEXT("How minimized windows arranged"),
SM_CXMINIMIZED, TEXT("SM_CXMINIMIZED"), TEXT("Minimized window width"),
SM_CYMINIMIZED, TEXT("SM_CYMINIMIZED"), TEXT("Minimized window height"),
SM_CXMAXTRACK, TEXT("SM_CXMAXTRACK"), TEXT("Maximum draggable width"),
SM_CYMAXTRACK, TEXT("SM_CYMAXTRACK"), TEXT("Maximum draggable height"),
SM_CXMAXIMIZED, TEXT("SM_CXMAXIMIZED"), TEXT("Width of maximized window"),
SM_CYMAXIMIZED, TEXT("SM_CYMAXIMIZED"), TEXT("Height of maximized window"),
SM_NETWORK, TEXT("SM_NETWORK"), TEXT("Network present flag"),
SM_CLEANBOOT, TEXT("SM_CLEANBOOT"), TEXT("How system was booted"),
SM_CXDRAG, TEXT("SM_CXDRAG"), TEXT("Avoid drag x tolerance"),
SM_CYDRAG, TEXT("SM CYDRAG"), TEXT("Avoid drag y tolerance"),
SM_SHOWSOUNDS, TEXT("SM_SHOWSOUNDS"), TEXT("Present sounds visually"),
SM_CXMENUCHECK, TEXT("SM_CXMENUCHECK"), TEXT("Menu check-mark width"),
SM_CYMENUCHECK, TEXT("SM_CYMENUCHECK"), TEXT("Menu check-mark height"),
SM_SLOWMACHINE, TEXT("SM_SLOWMACHINE"), TEXT("Slow processor flag"),
SM_MIDEASTENABLED, TEXT("SM_MIDEASTENABLED"), TEXT("Hebrew and Arabic enabled flag"),
SM_MOUSEWHEELPRESENT, TEXT("SM_MOUSEWHEELPRESENT"), TEXT("Mouse wheel present flag"),
SM_XVIRTUALSCREEN, TEXT("SM_XVIRTUALSCREEN"), TEXT("Virtual screen x origin"),
SM_YVIRTUALSCREEN, TEXT("SM_YVIRTUALSCREEN"), TEXT("Virtual screen y origin"),
SM_CXVIRTUALSCREEN, TEXT("SM_CXVIRTUALSCREEN"), TEXT("Virtual screen width"),
SM_CYVIRTUALSCREEN, TEXT("SM_CYVIRTUALSCREEN"), TEXT("Virtual screen height"),
SM_CMONITORS, TEXT("SM_CMONITORS"), TEXT("Number of monitors"),
SM_SAMEDISPLAYFORMAT, TEXT("SM_SAMEDISPLAYFORMAT"), TEXT("Same color format flag")
};
#endif

Answer 1

MSDN about UpdateWindow:
The function sends a WM_PAINT message directly to the window procedure
of the specified window, bypassing the application queue. If the
update region is empty, no message is sent.
To force an update you can manually set the update region with
InvalidateRect(hwnd, NULL, TRUE)
This adds a WM_PAINT message to the message queue of hwnd. If you want to force an instant repaint you can call UpdateWindow right afterwards but that shouldn't be necessary.