Make WebDriver DPI scaling-aware.

[grokys]

This PR makes the WebDriver process DPI scaling aware. If this is not done then capturing screenshots will not work on monitors with scaling other than 100% and sometimes element positioning is miscalculated.

It also makes the test application DPI-aware as it needs to communicate the current scaling factor to the unit tests. The test application exposes its current DPI scaling in the status bar, and this is read in tests that require checking bounds/positions.

Note that WinAppDriver is also DPI aware so this should also help match the behavior there.

[aristotelos]

Looks good, but I have a question whether this is needed for the web API.

[grokys]

Yep, even though it's a web API it interacts with the screen. And every process that interacts with the screen on Windows is subject to translation of screen coordinates if it's not per-monitor DPI aware. A couple of examples:

• The WebDriver reads the mouse position via Mouse.Position which calls GetCursorPos. If the process isn't DPI aware this can result in the coordinates being translated ¹
• If you add some UITests for the screenshot feature, you will find that they capture an empty image on monitors where scaling is not 100% unless the process is made DPI-aware

I've found some relevant Microsoft documentation for this:

Scaling in UI Automation Clients

The advice there is:

First, make the client application dpi-aware. To do this, call the SetProcessDPIAware function at startup. This function makes the entire process dpi-aware, meaning that all windows that belong to the process are unscaled.

The client application in this case being the WebDriver. This PR doesn't call SetProcessDPIAware though, it uses the alternative (and simpler IMO) method of Setting default awareness with the application manifest.

Footnotes

1. The documentation for GetCursorPos is confusing here. It says "The cursor position is always specified in screen coordinates and is not affected by the mapping mode of the window that contains the cursor." but the documentation for scaling in UI automation contradicts this by saying "For example, typically, the GetCursorPos function returns the logical coordinates". Perhaps "mapping mode" is different from logical coordinates, not sure. One can easily confirm that it uses logical coordinates though by using the API from a process that is not DPI aware. ↩