Position-independent font shaper caching (#8555)

Use relative cluster positioning to allow identical texts runs in
different row positions to share the same cache entry.

I am opening this PR clean w/o the cache size change. There could be
some benefit to a larger 256->512 shaper cache, but this still performs
amazingly well and I don't know the full memory impacts of moving the
cache size up.


https://github.com/ghostty-org/ghostty/discussions/8547#discussioncomment-14329590

src/font/shape.zig

@@ -38,9 +38,11 @@ pub const Shaper = switch (options.backend) {
 /// for a shaping call. Note all terminal cells may be present; only
 /// cells that have a glyph that needs to be rendered.
 pub const Cell = struct {
-    /// The column that this cell occupies. Since a set of shaper cells is
+    /// The X position of this shaper cell relative to the offset of the
-    /// always on the same line, only the X is stored. It is expected the
+    /// run. Because runs are always within a single row, it is expected
-    /// caller has access to the original screen cell.
+    /// that the caller can reconstruct the full position of the cell by
+    /// using the known Y position of the cell and adding the X position
+    /// to the run offset.
     x: u16,
 
     /// An additional offset to apply to the rendering.

src/font/shaper/run.zig

@@ -17,9 +17,15 @@ pub const TextRun = struct {
     /// lower the chance of hash collisions if they become a problem. If
     /// there are hash collisions, it would result in rendering issues but
     /// the core data would be correct.
+    ///
+    /// The hash is position-independent within the row by using relative
+    /// cluster positions. This allows identical runs in different positions
+    /// to share the same cache entry, improving cache efficiency.
     hash: u64,
 
-    /// The offset in the row where this run started
+    /// The offset in the row where this run started. This is added to the
+    /// X position of the final shaped cells to get the absolute position
+    /// in the row where they belong.
     offset: u16,
 
     /// The total number of cells produced by this run.
@@ -77,7 +83,11 @@ pub const RunIterator = struct {
         // Go through cell by cell and accumulate while we build our run.
         var j: usize = self.i;
         while (j < max) : (j += 1) {
-            const cluster = j;
+            // Use relative cluster positions (offset from run start) to make
+            // the shaping cache position-independent. This ensures that runs
+            // with identical content but different starting positions in the
+            // row produce the same hash, enabling cache reuse.
+            const cluster = j - self.i;
             const cell = &cells[j];
 
             // If we have a selection and we're at a boundary point, then

src/renderer/generic.zig

@@ -2528,9 +2528,11 @@ pub fn Renderer(comptime GraphicsAPI: type) type {
                                 break :cache cells;
                             };
 
+                        const cells = shaper_cells.?;
+
                         // Advance our index until we reach or pass
                         // our current x position in the shaper cells.
-                        while (shaper_cells.?[shaper_cells_i].x < x) {
+                        while (run.offset + cells[shaper_cells_i].x < x) {
                             shaper_cells_i += 1;
                         }
                     }
@@ -2769,13 +2771,13 @@ pub fn Renderer(comptime GraphicsAPI: type) type {
                         // If we encounter a shaper cell to the left of the current
                         // cell then we have some problems. This logic relies on x
                         // position monotonically increasing.
-                        assert(cells[shaper_cells_i].x >= x);
+                        assert(run.offset + cells[shaper_cells_i].x >= x);
 
                         // NOTE: An assumption is made here that a single cell will never
                         // be present in more than one shaper run. If that assumption is
                         // violated, this logic breaks.
 
-                        while (shaper_cells_i < cells.len and cells[shaper_cells_i].x == x) : ({
+                        while (shaper_cells_i < cells.len and run.offset + cells[shaper_cells_i].x == x) : ({
                             shaper_cells_i += 1;
                         }) {
                             self.addGlyph(