T22: the macOS menu bar holds the PRD's menus, each item showing its key; Quit, Select All, Bigger, Smaller, Reset Size and Minimize take Ghostty's keys, Ctrl-Shift off macOS where Ctrl with a letter types a control byte; the font's size moves for every session, unsaved; Select All runs from the first character in the history to the last, as Ghostty's; shot lists the menus and says whether the app was active at each key

src/term/zoom.rs

@@ -0,0 +1,54 @@
+use gpui::{App, Global};
+
+use crate::settings::{FONT_SIZE, SettingsStore};
+
+/// Points the terminal's font stands above the Size setting, below it when negative; Bigger and Smaller move it, nothing saves it.
+#[derive(Default)]
+pub struct Enlarged(f32);
+
+impl Global for Enlarged {}
+
+/// The size the terminal draws its font at: the setting, moved, inside the range.
+pub fn size(cx: &App) -> f32 {
+    shown(SettingsStore::get(cx).font_size, cx.global::<Enlarged>().0)
+}
+
+/// Moves the font `by` points, or back to the setting when `by` is none.
+pub fn step(by: Option<f32>, cx: &mut App) {
+    let setting = SettingsStore::get(cx).font_size;
+    let enlarged = by.map_or(0.0, |by| moved(setting, cx.global::<Enlarged>().0, by));
+    cx.set_global(Enlarged(enlarged));
+    log::info!(
+        "zoom: the terminal's font is {} points, {enlarged:+} on the setting",
+        shown(setting, enlarged)
+    );
+}
+
+fn shown(setting: f32, enlarged: f32) -> f32 {
+    (setting + enlarged).clamp(*FONT_SIZE.start(), *FONT_SIZE.end())
+}
+
+/// What `enlarged` becomes once the font moves `by` points from where it shows.
+fn moved(setting: f32, enlarged: f32, by: f32) -> f32 {
+    shown(shown(setting, enlarged) + by, 0.0) - setting
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn a_step_moves_the_font_inside_the_range() {
+        assert_eq!(moved(13.0, 0.0, 1.0), 1.0);
+        assert_eq!(moved(13.0, 2.0, -1.0), 1.0);
+        assert_eq!(moved(71.0, 0.0, 1.0), 1.0);
+        assert_eq!(moved(72.0, 0.0, 1.0), 0.0, "the top holds");
+        assert_eq!(moved(6.0, 0.0, -1.0), 0.0, "the bottom holds");
+        assert_eq!(
+            shown(70.0, 5.0),
+            72.0,
+            "a larger setting under a zoom stays in range"
+        );
+        assert_eq!(moved(70.0, 5.0, -1.0), 1.0, "a step starts from what shows");
+    }
+}