a11y: magnifier: change zoom stepping to multiplicative, bound max, add presets

1. Change magnifier zoom stepping from additive to multiplicative,
   because zoom is a ratio, not a difference, and human perception is
   logarithmic (browser zoom steps, map zoom levels, microscope powers).

2. Add `pub const ZoomConfig::ZOOM_INCREMENT_PRESETS` with two
   additional presets: 10% (macOS, Cinnamon) and 75% (for users at
   70-90x zoom).

3. Bound the zoom level to a generous maximum, beyond what anyone is expected
   to need, to prevent runaway / unrecoverable levels (#1230).

Full background and rationale at #2615.

I used AI to assist with research and issue writeup. Code is 100% mine.

- Closes #2615.
- Fixes #1239.
- Partially addresses #1230 and #1237.

cosmic-comp-config/src/lib.rs

@@ -198,6 +198,10 @@ pub struct ZoomConfig {
     pub enable_mouse_zoom_shortcuts: bool,
 }
 
+impl ZoomConfig {
+    pub const ZOOM_INCREMENT_PRESETS: &[u32] = &[10, 25, 50, 75, 100, 150, 200];
+}
+
 impl Default for ZoomConfig {
     fn default() -> Self {
         ZoomConfig {

src/input/actions.rs

@@ -29,6 +29,8 @@ use std::{os::unix::process::CommandExt, thread};
 
 use super::gestures;
 
+const MAX_ZOOM: f64 = 256.0;
+
 fn propagate_by_default(action: &shortcuts::Action) -> bool {
     matches!(
         action,
@@ -1116,7 +1118,13 @@ impl State {
         }
 
         if zoom_seat == *seat {
-            let new_level = (current_level + change).max(1.0);
+            let factor = 1.0 + change.abs();
+            let new_level = if change < 0. {
+                current_level / factor
+            } else {
+                current_level * factor
+            }
+            .clamp(1.0, MAX_ZOOM);
             shell.trigger_zoom(
                 seat,
                 Some(&output),

src/shell/zoom.rs

@@ -7,7 +7,7 @@ use cosmic::{
     theme,
     widget::{self, icon::Named},
 };
-use cosmic_comp_config::ZoomMovement;
+use cosmic_comp_config::{ZoomConfig, ZoomMovement};
 use cosmic_config::ConfigSet;
 use keyframe::{ease, functions::Linear};
 use smithay::{
@@ -447,7 +447,7 @@ pub enum MenuMessage {
 
 impl ZoomProgram {
     pub fn new(level: f64, movement: ZoomMovement, increment: u32) -> Self {
-        let mut increments = vec![25, 50, 100, 150, 200];
+        let mut increments = ZoomConfig::ZOOM_INCREMENT_PRESETS.to_vec();
         if !increments.contains(&increment) {
             increments.push(increment);
         }
@@ -828,7 +828,7 @@ impl Program for ZoomProgram {
                 if let Some(pos) = self.increments.iter().position(|val| *val == increment) {
                     self.increment_idx = pos;
                 } else {
-                    let mut increments = vec![25, 50, 100, 150, 200];
+                    let mut increments = ZoomConfig::ZOOM_INCREMENT_PRESETS.to_vec();
                     if !increments.contains(&increment) {
                         increments.push(increment);
                     }