Make fallback alignment behaviour configurable

Signed-off-by: Nico Burns <[email protected]>

examples/swash_render/src/main.rs

@@ -165,7 +165,11 @@ fn main() {
 
     // Perform layout (including bidi resolution and shaping) with start alignment
     layout.break_all_lines(max_advance);
-    layout.align(max_advance, Alignment::Start);
+    layout.align(
+        max_advance,
+        Alignment::Start,
+        false, /* align_when_overflowing */
+    );
 
     // Create image to render into
     let width = layout.width().ceil() as u32 + (padding * 2);

examples/tiny_skia_render/src/main.rs

@@ -96,7 +96,11 @@ fn main() {
 
     // Perform layout (including bidi resolution and shaping) with start alignment
     layout.break_all_lines(max_advance);
-    layout.align(max_advance, Alignment::Start);
+    layout.align(
+        max_advance,
+        Alignment::Start,
+        false, /* align_when_overflowing */
+    );
     let width = layout.width().ceil() as u32;
     let height = layout.height().ceil() as u32;
     let padded_width = width + padding * 2;

parley/src/layout/alignment.rs

@@ -8,6 +8,7 @@ pub(crate) fn align<B: Brush>(
     layout: &mut LayoutData<B>,
     alignment_width: Option<f32>,
     alignment: Alignment,
+    align_when_overflowing: bool,
 ) {
     let alignment_width = alignment_width.unwrap_or_else(|| {
         let max_line_length = layout
@@ -27,6 +28,10 @@ pub(crate) fn align<B: Brush>(
         // Compute free space.
         let free_space = alignment_width - line.metrics.advance + line.metrics.trailing_whitespace;
 
+        if !align_when_overflowing && free_space <= 0.0 {
+            continue;
+        }
+
         match alignment {
             Alignment::Start => {
                 // Do nothing

parley/src/layout/editor.rs

@@ -1038,7 +1038,11 @@ where
         }
         self.layout = builder.build(&self.buffer);
         self.layout.break_all_lines(self.width);
-        self.layout.align(self.width, self.alignment);
+        self.layout.align(
+            self.width,
+            self.alignment,
+            false, /* align_when_overflowing */
+        );
         self.selection = self.selection.refresh(&self.layout);
         self.layout_dirty = false;
         self.generation.nudge();

parley/src/layout/mod.rs

@@ -144,8 +144,22 @@ impl<B: Brush> Layout<B> {
 
     // Apply to alignment to layout relative to the specified container width. If container_width is not
     // specified then the max line length is used.
-    pub fn align(&mut self, container_width: Option<f32>, alignment: Alignment) {
-        align(&mut self.data, container_width, alignment);
+    //
+    // If `align_when_overflowing` is set to `true` then `Center` and `End` alignment will apply even if
+    // the line contents are wider than the `container_width`. If it is set to `false` then all overflowing
+    // lines will be `Start` aligned.
+    pub fn align(
+        &mut self,
+        container_width: Option<f32>,
+        alignment: Alignment,
+        align_when_overflowing: bool,
+    ) {
+        align(
+            &mut self.data,
+            container_width,
+            alignment,
+            align_when_overflowing,
+        );
     }
 
     /// Returns the index and `Line` object for the line containing the