How to align window to windows' taskbar

Question

I'm trying to position my Win32 API window alongside (e.g. left-sided vertical) taskbar. My display has 2560x1600 resolution and 144 DPI. I had some problems with DPI-aware apps previously, so maybe I still don't undestand some DPI-related things. For example, now I set DPI-awarness both programmatically - setting the DPI_AWARENESS_CONTEXT_PER_MONITOR_AWARE_V2 option through the Win32 API, and adding two lines (to support Windows 7-10) to the project's manifest file:

<dpiAware xmlns="http://schemas.microsoft.com/SMI/2005/WindowsSettings">True/PM</dpiAware>
<dpiAwareness xmlns="http://schemas.microsoft.com/SMI/2016/WindowsSettings">PerMonitorV2</dpiAwareness>

Here is a code snippet, that shows how I'm setting a window's position:

RECT taskbarRect = {0, 0, 0, 0};
HWND taskbarHandle = FindWindow(L"Shell_TrayWnd", NULL);
if (taskbarHandle) {
 GetWindowRect(taskbarHandle, &taskbarRect);
} else {...}
RECT notificationWindowRect; // Here is the RECT for window, I'm trying to reposition.
GetWindowRect(notificationWindowHandle, &notificationWindowRect);
LONG newX = 0;
LONG newY = 0;
bool taskbarIsVertical = (taskbarRect.Height() > taskbarRect.Width());
if (taskbarRect.left == 0 && taskbarIsVertical) { // left vertical taskbar
 newX = taskbarRect.right;
 newY = taskbarRect.bottom - notificationWindowRect.Height();
} else {...}
SetWindowPos(notificationWindowHandle, NULL, newX, newY, 0, 0, SWP_NOSIZE | SWP_NOZORDER | SWP_NOACTIVATE);

When system scaling is set to 100%, it's almost working - taskbarRect has a width of 63, but still there is a gap of a few pixels between the taskbar's right side and my window's left side. Note, that my window has the popup style and has no borders.

However, the main problem happens, when I set Windows' scaling to 150%. From the one hand, the taskbarRect's width becomes equal to 94, which I suppose is correct because 63 * 1.5 == 94. On the other hand, my window becomes hidden a little bit from the left side by the taskbar. To handle that, I need to add 65 pixels:

newX = taskbarRect.right + 65;

I don't understand where this 65-pixel shift appears from, and why it is exactly 65 pixels.

• Jonathan Potter: Windows have invisible borders these days, trying to align two windows is quite difficult and varies between OS versions and (as you've seen) different DPIs. Black magic basically.
• Anders: Don't use Shell_TrayWnd, use the monitor work area.
• zett42: To get the visual size (i. e. without the drop shadow) of a window, call DwmGetWindowAttribute() with DWMWA_EXTENDED_FRAME_BOUNDS. This API has been available since Windows Vista.
• dialer: Your code looks like it should work to some extent (see below). And I cannot reproduce the issue. Your code perfecly (no gap) positions a WS_POPUP window on my system with a left taskbar on 150% dpi (also no gap on 100%). Some guesses: Do you have multiple monitors? Maybe something gets confused -- this is also why Shell_TrayWnd is a bad idea - multiple monitors means multiple taskbars, do what @Anders suggested. Or maybe you are running that code in the wrong place, maybe it's too early and then accidentally some other code repositions the window?
• qloq: @dialer. No, I don't use multiple monitors now. Maybe I will test it after this, more simple, case will be solved. There is no code for reposition after code, that I mentioned in the first post. However I found new problems - something automatically resizes my window to minimize gaps between controls and window' borders, so I need to insert dummy controls to keep those gaps. Maybe I have some fundamental bugs in my code, e.g. in CreateWindow or RegisterClassEx.