Allow the agent panel font size to be customized (#29954)

You can set `agent_font_size` as a top-level settings key. You can also
use `zed::IncreaseBufferFontSize` and `zed::DecreaseBufferFontSize` and
`zed::ResetBufferFontSize` the agent panel is focused via the standard
bindings to adjust the agent font size. In the future, it might make
sense to rename these actions to be more general since "buffer" is now a
bit of a misnomer. 🍐'd with @mikayla-maki

Release Notes:

- N/A

---------

Co-authored-by: Mikayla Maki <mikayla.c.maki@gmail.com>

crates/theme/src/settings.rs

@@ -106,6 +106,8 @@ pub struct ThemeSettings {
     ///
     /// The terminal font family can be overridden using it's own setting.
     pub buffer_font: Font,
+    /// The agent font size. Determines the size of text in the agent panel.
+    agent_font_size: Pixels,
     /// The line height for buffers, and the terminal.
     ///
     /// Changing this may affect the spacing of some UI elements.
@@ -251,6 +253,11 @@ pub(crate) struct UiFontSize(Pixels);
 
 impl Global for UiFontSize {}
 
+#[derive(Default)]
+pub(crate) struct AgentFontSize(Pixels);
+
+impl Global for AgentFontSize {}
+
 /// Represents the selection of a theme, which can be either static or dynamic.
 #[derive(Clone, Debug, Serialize, Deserialize, JsonSchema, PartialEq, Eq)]
 #[serde(untagged)]
@@ -409,6 +416,9 @@ pub struct ThemeSettingsContent {
     #[serde(default)]
     #[schemars(default = "default_font_features")]
     pub buffer_font_features: Option<FontFeatures>,
+    /// The font size for the agent panel.
+    #[serde(default)]
+    pub agent_font_size: Option<f32>,
     /// The name of the Zed theme to use.
     #[serde(default)]
     pub theme: Option<ThemeSelection>,
@@ -579,6 +589,15 @@ impl ThemeSettings {
         clamp_font_size(font_size)
     }
 
+    /// Returns the UI font size.
+    pub fn agent_font_size(&self, cx: &App) -> Pixels {
+        let font_size = cx
+            .try_global::<AgentFontSize>()
+            .map(|size| size.0)
+            .unwrap_or(self.agent_font_size);
+        clamp_font_size(font_size)
+    }
+
     /// Returns the buffer font size, read from the settings.
     ///
     /// The real buffer font size is stored in-memory, to support temporary font size changes.
@@ -746,6 +765,26 @@ pub fn reset_ui_font_size(cx: &mut App) {
     }
 }
 
+/// Sets the adjusted UI font size.
+pub fn adjust_agent_font_size(cx: &mut App, mut f: impl FnMut(&mut Pixels)) {
+    let agent_font_size = ThemeSettings::get_global(cx).agent_font_size(cx);
+    let mut adjusted_size = cx
+        .try_global::<AgentFontSize>()
+        .map_or(agent_font_size, |adjusted_size| adjusted_size.0);
+
+    f(&mut adjusted_size);
+    cx.set_global(AgentFontSize(clamp_font_size(adjusted_size)));
+    cx.refresh_windows();
+}
+
+/// Resets the UI font size to the default value.
+pub fn reset_agent_font_size(cx: &mut App) {
+    if cx.has_global::<AgentFontSize>() {
+        cx.remove_global::<AgentFontSize>();
+        cx.refresh_windows();
+    }
+}
+
 /// Ensures font size is within the valid range.
 pub fn clamp_font_size(size: Pixels) -> Pixels {
     size.max(MIN_FONT_SIZE)
@@ -789,6 +828,7 @@ impl settings::Settings for ThemeSettings {
             },
             buffer_font_size: defaults.buffer_font_size.unwrap().into(),
             buffer_line_height: defaults.buffer_line_height.unwrap(),
+            agent_font_size: defaults.agent_font_size.unwrap().into(),
             theme_selection: defaults.theme.clone(),
             active_theme: themes
                 .get(defaults.theme.as_ref().unwrap().theme(*system_appearance))
@@ -891,6 +931,12 @@ impl settings::Settings for ThemeSettings {
             );
             this.buffer_font_size = this.buffer_font_size.clamp(px(6.), px(100.));
 
+            merge(
+                &mut this.agent_font_size,
+                value.agent_font_size.map(Into::into),
+            );
+            this.agent_font_size = this.agent_font_size.clamp(px(6.), px(100.));
+
             merge(&mut this.buffer_line_height, value.buffer_line_height);
 
             // Clamp the `unnecessary_code_fade` to ensure text can't disappear entirely.