Fix DPI with 0 width/hight reported by xorg (#544)

* Fix DPI with 0 width/hight reported by xorg

* Add `WINIT_HIDPI_FACTOR` env variable

It is now possible to override the DPI factor using the
`WINIT_HIDPI_FACTOR` environment variable on X11.

The changelog also has been updated to introduce all current changes
made.

* Add documentation for the environment variable

* Fix nitpicks

* Learning the alphabet

* Panic with error message if DPI env var is <= 0

CHANGELOG.md

@@ -3,6 +3,8 @@
 - On X11, the `Moved` event is no longer sent when the window is resized without changing position.
 - `MouseCursor` and `CursorState` now implement `Default`.
 - `WindowBuilder::with_resizable` implemented for Windows.
+- On X11, if width or height is reported as 0, the DPI is now 1.0 instead of +inf.
+- On X11, the environment variable `WINIT_HIDPI_FACTOR` has been added for overriding DPI factor.
 
 # Version 0.15.0 (2018-05-22)
 

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

@@ -1,4 +1,5 @@
-use std::slice;
+use std::{env, slice};
+use std::str::FromStr;
 
 use super::*;
 use super::ffi::{
@@ -53,6 +54,22 @@ 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
+    if let Ok(dpi_factor_str) = env::var("WINIT_HIDPI_FACTOR") {
+        if let Ok(dpi_factor) = f64::from_str(&dpi_factor_str) {
+            if dpi_factor <= 0. {
+                panic!("Expected `WINIT_HIDPI_FACTOR` to be bigger than 0, got '{}'", dpi_factor);
+            }
+
+            return dpi_factor;
+        }
+    }
+
+    // See http://xpra.org/trac/ticket/728 for more information
+    if width_mm == 0 || width_mm == 0 {
+        return 1.0;
+    }
+
     let ppmm = (
         (width_px as f64 * height_px as f64) / (width_mm as f64 * height_mm as f64)
     ).sqrt();

src/window.rs

@@ -344,6 +344,10 @@ impl Window {
     /// Returns the ratio between the backing framebuffer resolution and the
     /// window size in screen pixels. This is typically one for a normal display
     /// and two for a retina display.
+    ///
+    /// ## Platform-specific
+    /// On X11 the DPI factor can be overridden using the `WINIT_HIDPI_FACTOR` environment
+    /// variable.
     #[inline]
     pub fn hidpi_factor(&self) -> f32 {
         self.window.hidpi_factor()
@@ -467,6 +471,10 @@ impl MonitorId {
     }
 
     /// Returns the ratio between the monitor's physical pixels and logical pixels.
+    ///
+    /// ## Platform-specific
+    /// On X11 the DPI factor can be overridden using the `WINIT_HIDPI_FACTOR` environment
+    /// variable.
     #[inline]
     pub fn get_hidpi_factor(&self) -> f32 {
         self.inner.get_hidpi_factor()