This bug was reported both in T88384 and T86804.

Our placement and sizing of windows can be off by a small number of pixels because Windows 10 includes a thin invisible border (typically 7 pixels at left, right, bottom, but not at top) which we don't take into account.

Weirdest example is in trying to make a window at exactly a monitor's left edge. Passing 0 as "left" to CreateWindowExW() will create a window that is 7 pixels from the edge instead. AdjustWindowRectEx() does properly adjust for this and gives us the proper value to use, which is -7. But we clamp that to values within the monitor bounds.

This patch fixes this issue properly by first clamping our requested values to the monitor, then passing those values AdjustWindowRectEx() and then using those values for CreateWindowExW()

Following is a capture of me trying to place a window within a pixel or two of each edge:

But it ends up like this instead with all four edges incorrect.

After this patch is applied all the edges are perfect.

For testing, this bug report has good instructions: place a window in any specific size and location near edges, "Save Startup File", then close and relaunch.