Painting gives unusual results, black portions of window - c++

Question

I'm creating a win32 application in C++ and trying (unsuccessfully) to keep things somewhat object-orientated while still respecting the winapi message loop etc.
The problem I'm having is that my window seems to be drawing inconsistently and/or in a weird way that I have not encountered before. Eg:
This occurs when the window is resized. If I minimize the window and open it again, everything is black, apart from the buttons. Also, when moving my mouse from outside the window into the window, whatever my cursor was before getting inside the window is retained. For example, if there is a textbox behind the window, and I move my cursor from the textbox and into this window, the cursor remains the caret cursor.
I'm using this code to handle messages inside my WindowMain class:
std::map<HWND, WindowMain*> WindowMain::HWNDMap;
LRESULT CALLBACK WindowMain::WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam) {
if (HWNDMap.find(hWnd) == HWNDMap.end()) { return DefWindowProc(hWnd, msg, wParam, lParam); }
return HWNDMap[hWnd]->InstWndProc(hWnd, msg, wParam, lParam);
}
LRESULT CALLBACK WindowMain::InstWndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam) {
switch (msg) {
case WM_CLOSE:
PostQuitMessage(0);
break;
case WM_DESTROY:
DeleteObject(_hFont);
break;
default:
return DefWindowProc(hWnd, msg, wParam, lParam);
}
return 0;
}
The WindowMain::WndProc function is a static member of WindowMain. I use a std::map to map HWND to WindowMain* so that I can pass messages from the static method to the relevant WindowMain::InstWndProc. I feel like the problem might be here but I can't spot it.
In case it's relevant, this is the code I'm using to create the window:
WindowMain::WindowMain(HINSTANCE hInst) {
_isReady = false;
WNDCLASS wc;
memset(&wc, 0, sizeof(wc));
wc.lpfnWndProc = WndProc;
wc.lpszClassName = ClassName.c_str();
wc.hInstance = hInst;
wc.hbrBackground = (HBRUSH)(0);
if (!RegisterClass(&wc)) { return; }
_hWnd = CreateWindow(ClassName.c_str(), "AlchemyAC", WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, CW_USEDEFAULT, 400, 300, NULL, NULL, hInst, NULL);
if (!_hWnd) { return; }
_hBtnStart = CreateWindow("BUTTON", "Start", WS_TABSTOP | WS_VISIBLE | WS_CHILD, 10, 10, 100, 24, _hWnd, NULL, hInst, NULL);
_hBtnStop = CreateWindow("BUTTON", "Stop", WS_TABSTOP | WS_VISIBLE | WS_CHILD, 10, 39, 100, 24, _hWnd, NULL, hInst, NULL);
if (!_hBtnStart || !_hBtnStop) { return; }
GetObject(GetStockObject(DEFAULT_GUI_FONT), sizeof(LOGFONT), &_lf);
_hFont = CreateFont(_lf.lfHeight, _lf.lfWidth,
_lf.lfEscapement, _lf.lfOrientation, _lf.lfWeight,
_lf.lfItalic, _lf.lfUnderline, _lf.lfStrikeOut, _lf.lfCharSet,
_lf.lfOutPrecision, _lf.lfClipPrecision, _lf.lfQuality,
_lf.lfPitchAndFamily, _lf.lfFaceName);
SendMessage(_hBtnStart, WM_SETFONT, (WPARAM)_hFont, true);
SendMessage(_hBtnStop, WM_SETFONT, (WPARAM)_hFont, true);
EnableWindow(_hBtnStop, false);
HWNDMap[_hWnd] = this;
_isReady = true;
}
My question is, why is this strange painting issue occurring? I'm mainly interested in the odd thing that is happening with the cursor.
EDIT: I am using a while loop to translate and dispatch messages.