keymap: a layout switch forgets what letter positions typed, and Layout::forget_letters does so on request

crates/keymap/README.md

@@ -99,7 +99,9 @@ macOS it folds into `cmd` (`Chord::effective`).
 5. With Shift held on a Latin layout, a press that egui reports as the letter at its own
    position, because the character typed has no key (Dvorak's ⇧, types `<`), runs nothing when
    that position has been seen typing something other than its letter: once a `,` has been
-   typed, Ctrl+Shift+, never runs Ctrl+Shift+W.
+   typed, Ctrl+Shift+, never runs Ctrl+Shift+W. A change in the reported Latin status clears
+   what the positions typed; an app that learns of a layout switch itself calls
+   `Layout::forget_letters`.
 6. Labels show letters and digits by name, and a punctuation key as the character the layout
    types at its position when that character has no key of its own (`Ctrl+Ü` for Ctrl+[ on
    German).

crates/keymap/src/layout.rs

@@ -22,7 +22,8 @@
 //!    that has no key (Dvorak's ⇧, types `<`) as the key's position, W. With Shift held on a
 //!    Latin layout, such a press at a letter position that has been seen typing something other
 //!    than its letter resolves to nothing, so Ctrl+Shift+, never runs Ctrl+Shift+W once a `,`
-//!    has been typed.
+//!    has been typed. A change in the reported Latin status, or [`Layout::forget_letters`],
+//!    clears what the positions typed.
 //! 6. **Labels** show letters and digits by name, and a punctuation key as the character the
 //!    layout types at its position when that character has no key of its own: Ctrl+[ is `Ctrl+Ü`
 //!    on German and `Ctrl+Х` on Russian.
@@ -143,11 +144,23 @@ impl Layout {
     }
 
     /// Whether the layout is Latin, as the platform reports it; `None` when it cannot tell, so
-    /// that typing decides. Call again when the user switches layouts.
+    /// that typing decides. Call again when the user switches layouts. A report that differs
+    /// from the last one means another layout, so what letter positions typed is forgotten
+    /// ([`forget_letters`](Self::forget_letters)).
     pub fn set_latin(&mut self, latin: Option<bool>) {
+        if latin != self.reported_latin {
+            self.forget_letters();
+        }
         self.reported_latin = latin;
     }
 
+    /// Forgets what each letter position has been seen typing, for an app that learns of a
+    /// layout switch the platform does not report: until typing shows the new layout, Shift
+    /// presses at letter positions resolve as their letters again (rule 5).
+    pub fn forget_letters(&mut self) {
+        self.letters.clear();
+    }
+
     /// Whether letters are read as typed (rule 1): what the platform reported, else what a letter
     /// key typed last, else true.
     pub fn is_latin(&self) -> bool {

crates/keymap/tests/layout.rs

@@ -182,6 +182,29 @@ fn dvorak_shift_punctuation_never_runs_its_position() {
     assert_eq!(qwerty.resolve(&at("Cmd+Shift+E", Key::E)), Some(chord("Cmd+Shift+E")));
 }
 
+#[test]
+fn a_layout_switch_forgets_what_letter_positions_typed() {
+    // On Dvorak the W position types `,`; after a switch to QWERTY, Ctrl+Shift+W is W again.
+    let ctrl_shift_w = at("Cmd+Shift+W", Key::W);
+    let mut layout = Layout::new().with_latin(true);
+    layout.observe(Key::W, Mods::NONE, ",");
+    assert_eq!(layout.resolve(&ctrl_shift_w), None);
+    layout.set_latin(Some(true));
+    assert_eq!(layout.resolve(&ctrl_shift_w), None, "the same report is the same layout");
+
+    // Dvorak, then Russian, then QWERTY, as the platform reports them.
+    layout.set_latin(Some(false));
+    layout.set_latin(Some(true));
+    assert_eq!(layout.resolve(&ctrl_shift_w), Some(chord("Cmd+Shift+W")));
+
+    // An app that learns of a switch itself.
+    let mut unreported = Layout::new();
+    unreported.observe(Key::W, Mods::NONE, ",");
+    assert_eq!(unreported.resolve(&ctrl_shift_w), None);
+    unreported.forget_letters();
+    assert_eq!(unreported.resolve(&ctrl_shift_w), Some(chord("Cmd+Shift+W")));
+}
+
 #[test]
 fn labels_show_the_layouts_own_punctuation() {
     let mut german = Layout::new().with_char(Key::OpenBracket, 'ü').with_char(Key::Slash, '-').with_char(Key::CloseBracket, '+');