Add support for Windows 7 and 8

Cargo.toml

@@ -31,6 +31,7 @@ nix = "0.22.0"
 [target.'cfg(target_os="windows")'.dependencies]
 winapi = { version = "0.3.8", features = ["libloaderapi", "winuser", "windef", "minwindef", "guiddef", "combaseapi", "wingdi", "errhandlingapi"] }
 uuid = { version = "0.8", features = ["v4"], optional = true }
+libloading = "0.8.0"
 
 [target.'cfg(target_os="macos")'.dependencies]
 cocoa = "0.24.0"

src/win/dynamic_win_api.rs

@@ -0,0 +1,35 @@
+use libloading::{Library, Symbol};
+use winapi::shared::minwindef::{BOOL, UINT};
+use winapi::shared::windef::{DPI_AWARENESS_CONTEXT, HWND};
+
+/// Provides access to some Win32 API functions that are not available in older Windows versions.
+///
+/// This is better than eagerly linking to these functions because then the resulting binary
+/// wouldn't work *at all* in the older Windows versions, whereas with this approach, we can
+/// fall back to alternative logic or alternative values on a case-by-case basis.  
+pub struct DynamicWinApi {
+    user32_library: Library,
+}
+
+impl DynamicWinApi {
+    /// Loads the dynamic windows API, in particular "user32.dll".
+    pub fn load() -> Self {
+        unsafe { Self { user32_library: Library::new("user32.dll").unwrap() } }
+    }
+
+    /// Should be available from Windows 10 onwards.
+    pub fn set_process_dpi_awareness_context(
+        &self,
+    ) -> Option<Symbol<SetProcessDpiAwarenessContext>> {
+        unsafe { self.user32_library.get(b"SetProcessDpiAwarenessContext").ok() }
+    }
+
+    /// Should be available from Windows 10 onwards.
+    pub fn get_dpi_for_window(&self) -> Option<Symbol<GetDpiForWindow>> {
+        unsafe { self.user32_library.get(b"GetDpiForWindow").ok() }
+    }
+}
+
+type SetProcessDpiAwarenessContext = extern "stdcall" fn(value: DPI_AWARENESS_CONTEXT) -> BOOL;
+
+type GetDpiForWindow = extern "stdcall" fn(hwnd: HWND) -> UINT;

src/win/mod.rs

@@ -1,3 +1,4 @@
+mod dynamic_win_api;
 mod keyboard;
 mod window;
 

src/win/window.rs

@@ -4,16 +4,15 @@ use winapi::shared::windef::{HWND, RECT};
 use winapi::um::combaseapi::CoCreateGuid;
 use winapi::um::winuser::{
     AdjustWindowRectEx, CreateWindowExW, DefWindowProcW, DestroyWindow, DispatchMessageW,
-    GetDpiForWindow, GetMessageW, GetWindowLongPtrW, LoadCursorW, PostMessageW, RegisterClassW,
-    ReleaseCapture, SetCapture, SetProcessDpiAwarenessContext, SetTimer, SetWindowLongPtrW,
-    SetWindowPos, TranslateMessage, UnregisterClassW, CS_OWNDC, GET_XBUTTON_WPARAM, GWLP_USERDATA,
-    IDC_ARROW, MSG, SWP_NOMOVE, SWP_NOZORDER, WHEEL_DELTA, WM_CHAR, WM_CLOSE, WM_CREATE,
-    WM_DPICHANGED, WM_INPUTLANGCHANGE, WM_KEYDOWN, WM_KEYUP, WM_LBUTTONDOWN, WM_LBUTTONUP,
-    WM_MBUTTONDOWN, WM_MBUTTONUP, WM_MOUSEHWHEEL, WM_MOUSEMOVE, WM_MOUSEWHEEL, WM_NCDESTROY,
-    WM_RBUTTONDOWN, WM_RBUTTONUP, WM_SHOWWINDOW, WM_SIZE, WM_SYSCHAR, WM_SYSKEYDOWN, WM_SYSKEYUP,
-    WM_TIMER, WM_USER, WM_XBUTTONDOWN, WM_XBUTTONUP, WNDCLASSW, WS_CAPTION, WS_CHILD,
-    WS_CLIPSIBLINGS, WS_MAXIMIZEBOX, WS_MINIMIZEBOX, WS_POPUPWINDOW, WS_SIZEBOX, WS_VISIBLE,
-    XBUTTON1, XBUTTON2,
+    GetMessageW, GetWindowLongPtrW, LoadCursorW, PostMessageW, RegisterClassW, ReleaseCapture,
+    SetCapture, SetTimer, SetWindowLongPtrW, SetWindowPos, TranslateMessage, UnregisterClassW,
+    CS_OWNDC, GET_XBUTTON_WPARAM, GWLP_USERDATA, IDC_ARROW, MSG, SWP_NOMOVE, SWP_NOZORDER,
+    WHEEL_DELTA, WM_CHAR, WM_CLOSE, WM_CREATE, WM_DPICHANGED, WM_INPUTLANGCHANGE, WM_KEYDOWN,
+    WM_KEYUP, WM_LBUTTONDOWN, WM_LBUTTONUP, WM_MBUTTONDOWN, WM_MBUTTONUP, WM_MOUSEHWHEEL,
+    WM_MOUSEMOVE, WM_MOUSEWHEEL, WM_NCDESTROY, WM_RBUTTONDOWN, WM_RBUTTONUP, WM_SHOWWINDOW,
+    WM_SIZE, WM_SYSCHAR, WM_SYSKEYDOWN, WM_SYSKEYUP, WM_TIMER, WM_USER, WM_XBUTTONDOWN,
+    WM_XBUTTONUP, WNDCLASSW, WS_CAPTION, WS_CHILD, WS_CLIPSIBLINGS, WS_MAXIMIZEBOX, WS_MINIMIZEBOX,
+    WS_POPUPWINDOW, WS_SIZEBOX, WS_VISIBLE, XBUTTON1, XBUTTON2,
 };
 
 use std::cell::{Cell, RefCell};
@@ -35,6 +34,7 @@ use crate::{
 
 use super::keyboard::KeyboardState;
 
+use crate::win::dynamic_win_api::DynamicWinApi;
 #[cfg(feature = "opengl")]
 use crate::{gl::GlContext, window::RawWindowHandleWrapper};
 
@@ -681,15 +681,15 @@ impl Window<'_> {
             };
             *window_state.handler.borrow_mut() = Some(Box::new(handler));
 
-            // Only works on Windows 10 unfortunately.
-            SetProcessDpiAwarenessContext(
-                winapi::shared::windef::DPI_AWARENESS_CONTEXT_PER_MONITOR_AWARE,
-            );
+            let dynamic_win_api = DynamicWinApi::load();
+            if let Some(f) = dynamic_win_api.set_process_dpi_awareness_context() {
+                f(winapi::shared::windef::DPI_AWARENESS_CONTEXT_PER_MONITOR_AWARE);
+            }
 
             // Now we can get the actual dpi of the window.
             let new_rect = if let WindowScalePolicy::SystemScaleFactor = options.scale {
-                // Only works on Windows 10 unfortunately.
-                let dpi = GetDpiForWindow(hwnd);
+                let dpi =
+                    if let Some(f) = dynamic_win_api.get_dpi_for_window() { f(hwnd) } else { 96 };
                 let scale_factor = dpi as f64 / 96.0;
 
                 let mut window_info = window_state.window_info.borrow_mut();