terminal: introduce row bit for kitty virtual placeholders

src/terminal/PageList.zig

@@ -7,6 +7,7 @@ const std = @import("std");
 const Allocator = std.mem.Allocator;
 const assert = std.debug.assert;
 const fastmem = @import("../fastmem.zig");
+const kitty = @import("kitty.zig");
 const point = @import("point.zig");
 const pagepkg = @import("page.zig");
 const stylepkg = @import("style.zig");
@@ -1056,6 +1057,11 @@ const ReflowCursor = struct {
                 self.page_cell.style_id = id;
             }
 
+            // Copy Kitty virtual placeholder status
+            if (cell.codepoint() == kitty.graphics.unicode.placeholder) {
+                self.page_row.kitty_virtual_placeholder = true;
+            }
+
             self.cursorForward();
         }
 

src/terminal/Screen.zig

@@ -984,6 +984,16 @@ pub fn clearCells(
         if (cells.len == self.pages.cols) row.styled = false;
     }
 
+    if (row.kitty_virtual_placeholder and
+        cells.len == self.pages.cols)
+    {
+        for (cells) |c| {
+            if (c.codepoint() == kitty.graphics.unicode.placeholder) {
+                break;
+            }
+        } else row.kitty_virtual_placeholder = false;
+    }
+
     @memset(cells, self.blankCell());
 }
 

src/terminal/Terminal.zig

@@ -640,6 +640,12 @@ fn printCell(
         }
     }
 
+    // If this is a Kitty unicode placeholder then we need to mark the
+    // row so that the renderer can lookup rows with these much faster.
+    if (c == kitty.graphics.unicode.placeholder) {
+        self.screen.cursor.page_row.kitty_virtual_placeholder = true;
+    }
+
     // We check for an active hyperlink first because setHyperlink
     // handles clearing the old hyperlink and an optimization if we're
     // overwriting the same hyperlink.

src/terminal/page.zig

@@ -826,6 +826,9 @@ pub const Page = struct {
                         src_cell.style_id,
                     ) orelse src_cell.style_id;
                 }
+                if (src_cell.codepoint() == kitty.graphics.unicode.placeholder) {
+                    dst_row.kitty_virtual_placeholder = true;
+                }
             }
         }
 
@@ -913,6 +916,9 @@ pub const Page = struct {
                     dst.hyperlink = true;
                     dst_row.hyperlink = true;
                 }
+                if (src.codepoint() == kitty.graphics.unicode.placeholder) {
+                    dst_row.kitty_virtual_placeholder = true;
+                }
             }
         }
 
@@ -932,6 +938,7 @@ pub const Page = struct {
             src_row.grapheme = false;
             src_row.hyperlink = false;
             src_row.styled = false;
+            src_row.kitty_virtual_placeholder = false;
         }
     }
 
@@ -1029,6 +1036,16 @@ pub const Page = struct {
             if (cells.len == self.size.cols) row.styled = false;
         }
 
+        if (row.kitty_virtual_placeholder and
+            cells.len == self.size.cols)
+        {
+            for (cells) |c| {
+                if (c.codepoint() == kitty.graphics.unicode.placeholder) {
+                    break;
+                }
+            } else row.kitty_virtual_placeholder = false;
+        }
+
         // Zero the cells as u64s since empirically this seems
         // to be a bit faster than using @memset(cells, .{})
         @memset(@as([]u64, @ptrCast(cells)), 0);
@@ -1552,7 +1569,11 @@ pub const Row = packed struct(u64) {
     /// running program, or "unknown" if it was never set.
     semantic_prompt: SemanticPrompt = .unknown,
 
-    _padding: u24 = 0,
+    /// True if this row contains a virtual placeholder for the Kitty
+    /// graphics protocol. (U+10EEEE)
+    kitty_virtual_placeholder: bool = false,
+
+    _padding: u23 = 0,
 
     /// Semantic prompt type.
     pub const SemanticPrompt = enum(u3) {