Hello,
Emacs works perfectly on HiDPI monitor, I have patched ony two minor things:
1) declare dpi-awareness in manifest files, so Windows will not enable
"compatibility mode" and fonts now look crisp by default
2) remove unneeded OFN_ENABLEHOOK flag passed to GetOpenFileName, which
allow OS to show modern "open file" dialog. Legacy dialog not support HiDPI
mode and show too small icons.
Let me know if I miss something and/or select wrong place to report it

---

diff --git a/nt/emacs-x64.manifest b/nt/emacs-x64.manifestindex
5434e29..3cf56dd 100644--- a/nt/emacs-x64.manifest+++
b/nt/emacs-x64.manifest@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8" standalone="yes"?>-<assembly
xmlns="urn:schemas-microsoft-com:asm.v1"
manifestVersion="1.0">+<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"@@ -30,4 +30,9 @@
          <supportedOS Id="{4a2f28e3-53b9-4441-ba9c-d69d4a4a6e38}"/>
       </application>
     </compatibility>+  <asmv3:application>+    <asmv3:windowsSettings
xmlns="http://schemas.microsoft.com/SMI/2005/WindowsSettings">+
<dpiAware>true</dpiAware>+    </asmv3:windowsSettings>+
</asmv3:application>
 </assembly>diff --git a/nt/emacs-x86.manifest
b/nt/emacs-x86.manifestindex 7f09c56..d7053be 100644---
a/nt/emacs-x86.manifest+++ b/nt/emacs-x86.manifest@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8" standalone="yes"?>-<assembly
xmlns="urn:schemas-microsoft-com:asm.v1"
manifestVersion="1.0">+<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"@@ -30,4 +30,9 @@
         <supportedOS Id="{4a2f28e3-53b9-4441-ba9c-d69d4a4a6e38}"/>
       </application>
     </compatibility>+  <asmv3:application>+    <asmv3:windowsSettings
xmlns="http://schemas.microsoft.com/SMI/2005/WindowsSettings">+
<dpiAware>true</dpiAware>+    </asmv3:windowsSettings>+
</asmv3:application>
 </assembly>diff --git a/src/w32fns.c b/src/w32fns.cindex
ed11031..c04b9b0 100644--- a/src/w32fns.c+++ b/src/w32fns.c@@ -6680,7
+6680,7 @@ value of DIR as in previous invocations; this is standard
Windows behavior.  */)
 #endif
     file_details_w->nFilterIndex = NILP (only_dir_p) ? 1 : 2;
     file_details_w->Flags = (OFN_HIDEREADONLY | OFN_NOCHANGEDIR-
           | OFN_EXPLORER | OFN_ENABLEHOOK);+                 |
OFN_EXPLORER);
     if (!NILP (mustmatch))
       {
         /* Require that the path to the parent directory exists.
*/@@ -6707,7 +6707,7 @@ value of DIR as in previous invocations; this
is standard Windows behavior.  */)
     file_details_a->lpstrTitle = prompt_a;
     file_details_a->nFilterIndex = NILP (only_dir_p) ? 1 : 2;
     file_details_a->Flags = (OFN_HIDEREADONLY | OFN_NOCHANGEDIR-
           | OFN_EXPLORER | OFN_ENABLEHOOK);+                 |
OFN_EXPLORER);
     if (!NILP (mustmatch))
       {
         /* Require that the path to the parent directory exists.  */