renderer: apply glyph constraints when rasterizing glyphs

src/renderer/cell.zig

@@ -223,6 +223,60 @@ pub const FgMode = enum {
     powerline,
 };
 
+/// Returns the appropriate `constraint_width` for
+/// the provided cell when rendering its glyph(s).
+pub fn constraintWidth(cell_pin: terminal.Pin) u2 {
+    const cell = cell_pin.rowAndCell().cell;
+    const cp = cell.codepoint();
+
+    if (!ziglyph.general_category.isPrivateUse(cp) and
+        !ziglyph.blocks.isDingbats(cp))
+    {
+        return cell.gridWidth();
+    }
+
+    // If we are at the end of the screen it must be constrained to one cell.
+    if (cell_pin.x == cell_pin.node.data.size.cols - 1) return 1;
+
+    // If we have a previous cell and it was PUA then we need to
+    // also constrain. This is so that multiple PUA glyphs align.
+    // As an exception, we ignore powerline glyphs since they are
+    // used for box drawing and we consider them whitespace.
+    if (cell_pin.x > 0) prev: {
+        const prev_cp = prev_cp: {
+            var copy = cell_pin;
+            copy.x -= 1;
+            const prev_cell = copy.rowAndCell().cell;
+            break :prev_cp prev_cell.codepoint();
+        };
+
+        // We consider powerline glyphs whitespace.
+        if (isPowerline(prev_cp)) break :prev;
+
+        if (ziglyph.general_category.isPrivateUse(prev_cp)) {
+            return 1;
+        }
+    }
+
+    // If the next cell is whitespace, then
+    // we allow it to be up to two cells wide.
+    const next_cp = next_cp: {
+        var copy = cell_pin;
+        copy.x += 1;
+        const next_cell = copy.rowAndCell().cell;
+        break :next_cp next_cell.codepoint();
+    };
+    if (next_cp == 0 or
+        isSpace(next_cp) or
+        isPowerline(next_cp))
+    {
+        return 2;
+    }
+
+    // Must be constrained
+    return 1;
+}
+
 /// Returns the appropriate foreground mode for the given cell. This is
 /// meant to be called from the typical updateCell function within a
 /// renderer.

src/renderer/generic.zig

@@ -14,6 +14,7 @@ const Surface = @import("../Surface.zig");
 const link = @import("link.zig");
 const cellpkg = @import("cell.zig");
 const fgMode = cellpkg.fgMode;
+const constraintWidth = cellpkg.constraintWidth;
 const isCovering = cellpkg.isCovering;
 const imagepkg = @import("image.zig");
 const Image = imagepkg.Image;
@@ -26,6 +27,8 @@ const ArenaAllocator = std.heap.ArenaAllocator;
 const Terminal = terminal.Terminal;
 const Health = renderer.Health;
 
+const getConstraint = @import("../font/nerd_font_attributes.zig").getConstraint;
+
 const FileType = @import("../file_type.zig").FileType;
 
 const macos = switch (builtin.os.tag) {
@@ -3030,6 +3033,9 @@ pub fn Renderer(comptime GraphicsAPI: type) type {
                     .grid_metrics = self.grid_metrics,
                     .thicken = self.config.font_thicken,
                     .thicken_strength = self.config.font_thicken_strength,
+                    .cell_width = cell.gridWidth(),
+                    .constraint = getConstraint(cell.codepoint()),
+                    .constraint_width = constraintWidth(cell_pin),
                 },
             );