Here is what I did to solve the issue. It also works with older apps that don't handle high dpi scaling well.
First one must tell Windows to look for a manifest file for an application by default – a registry entry.
Next is to provide a manifest file for a given application – the executable file name followed by .manifest.
Open regedit and navigate to the registry key:
HKEY_LOCAL_MACHINE > SOFTWARE > Microsoft > Windows > CurrentVersion > SideBySide
Right-click, select NEW > DWORD (32 bit) Value
Type PreferExternalManifest, and then press ENTER.
Right-click PreferExternalManifest, and then click Modify.
Enter Value Data 1 and select Decimal.
Click OK. Exit Registry Editor.
The RDP app is %windir%\System32\mstsc.exe so the manifest file is mstsc.exe.manifest.
I can't attach the manifest file so here is the contents:
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0" xmlns:asmv3="urn:schemas-microsoft-com:asm.v3">
<dependency>
<dependentAssembly>
<assemblyIdentity
type="win32"
name="Microsoft.Windows.Common-Controls"
version="6.0.0.0" processorArchitecture="*"
publicKeyToken="6595b64144ccf1df"
language="*">
</assemblyIdentity>
</dependentAssembly>
</dependency>
<dependency>
<dependentAssembly>
<assemblyIdentity
type="win32"
name="Microsoft.VC90.CRT"
version="9.0.21022.8"
processorArchitecture="amd64"
publicKeyToken="1fc8b3b9a1e18e3b">
</assemblyIdentity>
</dependentAssembly>
</dependency>
<trustInfo xmlns="urn:schemas-microsoft-com:asm.v3">
<security>
<requestedPrivileges>
<requestedExecutionLevel
level="asInvoker"
uiAccess="false"/>
</requestedPrivileges>
</security>
</trustInfo>
<asmv3:application>
<asmv3:windowsSettings xmlns="http://schemas.microsoft.com/SMI/2005/WindowsSettings">
<ms_windowsSettings:dpiAware xmlns:ms_windowsSettings="http://schemas.microsoft.com/SMI/2005/WindowsSettings">false</ms_windowsSettings:dpiAware>
</asmv3:windowsSettings>
</asmv3:application>
</assembly>
Copy the above into notepad and save the file with the appropriate name to the same folder as the executable.