Fix: Handle AltGr on Windows

AltGr on windos sends a left control pressed followed by AltGr pressed
on both winit and Qt. On AltGr release it sends left control released
followed by AltGr released.

So unset left control on windows once AltGr is pressed to avoid treating
special characters entered via AltGr key combos to be treated as control
sequences -- leading to the input being ignored!

Also treat `Alt-Ctlr-X` as `AltGr-X` on windows.

Fixes: #2933

internal/core/input.rs

@@ -185,6 +185,7 @@ impl InternalKeyboardModifierState {
         if let Some(key_code) = text.chars().next() {
             match key_code {
                 key_codes::Alt => self.left_alt = pressed,
+                key_codes::AltGr => { /* fall through to special cases */ }
                 key_codes::Control => self.left_control = pressed,
                 key_codes::ControlR => self.right_control = pressed,
                 key_codes::Shift => self.left_shift = pressed,
@@ -193,10 +194,24 @@ impl InternalKeyboardModifierState {
                 key_codes::MetaR => self.right_meta = pressed,
                 _ => return None,
             };
+
             // Encoded keyboard modifiers must appear as individual key events. This could
             // be relaxed by implementing a string split, but right now WindowEvent::KeyPressed
             // holds only a single char.
             debug_assert_eq!(key_code.len_utf8(), text.len());
+
+            // Special cases:
+            #[cfg(target_os = "windows")]
+            {
+                if key_code == key_codes::AltGr {
+                    // Windows sends Ctrl followed by AltGr on AltGr. Disable the Ctrl again!
+                    self.left_control = false;
+                } else if self.left_control && self.left_alt {
+                    // Windows treats Ctrl-Alt as AltGr:
+                    self.left_control = false;
+                    self.left_alt = false;
+                }
+            }
         }
 
         Some(self)