image rendering and visual selection

src/glyph_shader.wgsl

@@ -157,6 +157,7 @@ struct GridParams {
     selection_start_row: i32,
     selection_end_col: i32,
     selection_end_row: i32,
+    selection_row_max_col: array<i32, 256>,
 }
 
 // GPUCell instance data (matches Rust GPUCell struct)
@@ -187,7 +188,7 @@ struct SpriteInfo {
 var<uniform> color_table: ColorTable;
 
 @group(1) @binding(1)
-var<uniform> grid_params: GridParams;
+var<storage, read> grid_params: GridParams;
 
 @group(1) @binding(2)
 var<storage, read> cells: array<GPUCell>;
@@ -277,6 +278,11 @@ fn is_cell_selected(col: u32, row: u32) -> bool {
     if grid_params.selection_start_col < 0 || grid_params.selection_start_row < 0 {
         return false;
     }
+
+    // Only highlight cells that have content in them or to their right on this row
+    if grid_params.selection_row_max_col[row] < 0 || col > u32(grid_params.selection_row_max_col[row]) {
+        return false;
+    }
     
     let sel_start_col = u32(grid_params.selection_start_col);
     let sel_start_row = u32(grid_params.selection_start_row);