On X11, make WINIT_HIDPI_FACTOR dominate Xft.dpi in some cases (#1354)

* On X11, make `WINIT_HIDPI_FACTOR` dominate `Xft.dpi` in some cases

This commit makes `WINIT_HIDPI_FACTOR` dominate `Xft.dpi` in general and
adds a special value `0` for `WINIT_HIDPI_FACTOR` to use winit computed
DPI factor with randr over Xft.dpi.

* Use `randr` instead of `0` for auto dpi scaling

* Update CHANGELOG

* blow up on wrong env var

* Allow empty string for env var

CHANGELOG.md

@@ -23,6 +23,8 @@
 - On Windows, fix `Window::set_visible` not setting internal flags correctly. This resulted in some weird behavior.
 - Add `DeviceEvent::ModifiersChanged`.
   - Deprecate `modifiers` fields in other events in favor of `ModifiersChanged`.
+- On X11, `WINIT_HIDPI_FACTOR` now dominates `Xft.dpi` when picking DPI factor for output.
+- On X11, add special value `randr` for `WINIT_HIDPI_FACTOR` to make winit use self computed DPI factor instead of the one from `Xft.dpi`.
 
 # 0.20.0 Alpha 5 (2019-12-09)
 

src/platform_impl/linux/x11/util/randr.rs

@@ -6,24 +6,17 @@ use super::{
 };
 use crate::{dpi::validate_hidpi_factor, platform_impl::platform::x11::VideoMode};
 
+/// Represents values of `WINIT_HIDPI_FACTOR`.
+pub enum EnvVarDPI {
+    Randr,
+    Scale(f64),
+    NotSet,
+}
+
 pub fn calc_dpi_factor(
     (width_px, height_px): (u32, u32),
     (width_mm, height_mm): (u64, u64),
 ) -> f64 {
-    // Override DPI if `WINIT_HIDPI_FACTOR` variable is set
-    let dpi_override = env::var("WINIT_HIDPI_FACTOR")
-        .ok()
-        .and_then(|var| f64::from_str(&var).ok());
-    if let Some(dpi_override) = dpi_override {
-        if !validate_hidpi_factor(dpi_override) {
-            panic!(
-                "`WINIT_HIDPI_FACTOR` invalid; DPI factors must be normal floats greater than 0. Got `{}`",
-                dpi_override,
-            );
-        }
-        return dpi_override;
-    }
-
     // See http://xpra.org/trac/ticket/728 for more information.
     if width_mm == 0 || height_mm == 0 {
         warn!("XRandR reported that the display's 0mm in size, which is certifiably insane");
@@ -107,16 +100,55 @@ impl XConnection {
             (*output_info).nameLen as usize,
         );
         let name = String::from_utf8_lossy(name_slice).into();
-        let hidpi_factor = if let Some(dpi) = self.get_xft_dpi() {
-            dpi / 96.
-        } else {
-            calc_dpi_factor(
+        // Override DPI if `WINIT_HIDPI_FACTOR` variable is set
+        let dpi_env = env::var("WINIT_HIDPI_FACTOR").ok().map_or_else(
+            || EnvVarDPI::NotSet,
+            |var| {
+                if var.to_lowercase() == "randr" {
+                    EnvVarDPI::Randr
+                } else if let Ok(dpi) = f64::from_str(&var) {
+                    EnvVarDPI::Scale(dpi)
+                } else if var.is_empty() {
+                    EnvVarDPI::NotSet
+                } else {
+                    panic!(
+                        "`WINIT_HIDPI_FACTOR` invalid; DPI factors must be either normal floats greater than 0, or `randr`. Got `{}`",
+                        var
+                    );
+                }
+            },
+        );
+
+        let hidpi_factor = match dpi_env {
+            EnvVarDPI::Randr => calc_dpi_factor(
                 ((*crtc).width as u32, (*crtc).height as u32),
                 (
                     (*output_info).mm_width as u64,
                     (*output_info).mm_height as u64,
                 ),
-            )
+            ),
+            EnvVarDPI::Scale(dpi_override) => {
+                if !validate_hidpi_factor(dpi_override) {
+                    panic!(
+                        "`WINIT_HIDPI_FACTOR` invalid; DPI factors must be either normal floats greater than 0, or `randr`. Got `{}`",
+                        dpi_override,
+                    );
+                }
+                dpi_override
+            }
+            EnvVarDPI::NotSet => {
+                if let Some(dpi) = self.get_xft_dpi() {
+                    dpi / 96.
+                } else {
+                    calc_dpi_factor(
+                        ((*crtc).width as u32, (*crtc).height as u32),
+                        (
+                            (*output_info).mm_width as u64,
+                            (*output_info).mm_height as u64,
+                        ),
+                    )
+                }
+            }
         };
 
         (self.xrandr.XRRFreeOutputInfo)(output_info);