Don't let AtomLayoutStyle override a per-widget atom gap

`AtomLayout::gap` is an `Option`, where `None` means "use the theme value".
`AtomLayoutStyle::apply` set it unconditionally, so it clobbered the gap a
widget had asked for. `DragValue` sets `gap(0.0)` to keep its prefix, value and
suffix flush, and lost it, which widened every `DragValue` in the demos.

`apply` now fills the gap in only when the widget left it unset. This also
reverts the snapshot updates that had captured the wider gap.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

crates/egui/src/atomics/atom_layout.rs

@@ -107,6 +107,16 @@ impl<'a> AtomLayout<'a> {
         self
     }
 
+    /// Set the gap between atoms, unless the caller already set one.
+    ///
+    /// Used to apply a [`crate::widget_style::AtomLayoutStyle`] without overriding a per-widget
+    /// [`Self::gap`].
+    #[inline]
+    pub(crate) fn gap_if_unset(mut self, gap: f32) -> Self {
+        self.gap = self.gap.or(Some(gap));
+        self
+    }
+
     /// Set the [`Frame`].
     #[inline]
     pub fn frame(mut self, frame: Frame) -> Self {

crates/egui/src/widget_style/mod.rs

@@ -101,6 +101,9 @@ impl Default for AtomLayoutStyle {
 
 impl AtomLayoutStyle {
     /// Apply this style to an [`AtomLayout`].
+    ///
+    /// A per-widget [`AtomLayout::gap`] wins over [`Self::gap`], so widgets like
+    /// [`crate::DragValue`] can pack their atoms tighter than the theme does.
     pub fn apply<'a>(&self, mut layout: AtomLayout<'a>) -> AtomLayout<'a> {
         layout.map_images(|image| {
             if image.image_options().tint == Color32::WHITE {
@@ -112,7 +115,7 @@ impl AtomLayoutStyle {
 
         let layout = layout
             .min_size(self.min_size)
-            .gap(self.gap)
+            .gap_if_unset(self.gap)
             .frame(self.frame)
             .fallback_font(self.text_style.font_id.clone())
             .fallback_text_color(self.text_style.color);

crates/egui_demo_app/tests/snapshots/imageviewer.png

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crates/egui_demo_lib/tests/snapshots/demos/Code Example.png

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