kitty img: make allowances for permanence of image attachments

An implementation detail in wezterm is that it doesn't model
image placements as a separate entity; they are all bound to
the image cells in the terminal model.

The semantics of the kitty image protocol are that placements
are "permanent" wrt. overwriting a cell with text, except for
the explicit EraseInLine/EraseInDisplay sequences that are
used for clearing.

This commit takes a pass at implementing that semantic in
the wezterm data model.

refs: #986

term/src/terminalstate/image.rs

@@ -40,7 +40,7 @@ pub struct ImageAttachParams {
     pub columns: Option<usize>,
     pub rows: Option<usize>,
 
-    pub image_id: u32,
+    pub image_id: Option<u32>,
     pub placement_id: Option<u32>,
 
     pub style: ImageAttachStyle,

term/src/terminalstate/iterm.rs

@@ -136,7 +136,7 @@ impl TerminalState {
             rows: None,
             data: image_data,
             style: ImageAttachStyle::Iterm,
-            image_id: 0,
+            image_id: None,
             placement_id: None,
             do_not_move_cursor: false,
         });

term/src/terminalstate/kitty.rs

@@ -124,7 +124,7 @@ impl TerminalState {
             z_index: placement.z_index.unwrap_or(0),
             columns: placement.columns.map(|x| x as usize),
             rows: placement.rows.map(|x| x as usize),
-            image_id,
+            image_id: Some(image_id),
             placement_id: placement.placement_id,
             do_not_move_cursor: placement.do_not_move_cursor,
         });

term/src/terminalstate/sixel.rs

@@ -121,7 +121,7 @@ impl TerminalState {
             data: image_data,
             style: ImageAttachStyle::Sixel,
             z_index: 0,
-            image_id: 0,
+            image_id: None,
             placement_id: None,
             do_not_move_cursor: false,
         });

termwiz/src/image.rs

@@ -90,7 +90,7 @@ pub struct ImageCell {
     display_offset_x: u32,
     display_offset_y: u32,
 
-    image_id: u32,
+    image_id: Option<u32>,
     placement_id: Option<u32>,
 }
 
@@ -100,7 +100,7 @@ impl ImageCell {
         bottom_right: TextureCoordinate,
         data: Arc<ImageData>,
     ) -> Self {
-        Self::with_z_index(top_left, bottom_right, data, 0, 0, 0, 0, None)
+        Self::with_z_index(top_left, bottom_right, data, 0, 0, 0, None, None)
     }
 
     pub fn with_z_index(
@@ -110,7 +110,7 @@ impl ImageCell {
         z_index: i32,
         display_offset_x: u32,
         display_offset_y: u32,
-        image_id: u32,
+        image_id: Option<u32>,
         placement_id: Option<u32>,
     ) -> Self {
         Self {
@@ -126,7 +126,11 @@ impl ImageCell {
     }
 
     pub fn matches_placement(&self, image_id: u32, placement_id: Option<u32>) -> bool {
-        self.image_id == image_id && self.placement_id == placement_id
+        self.image_id == Some(image_id) && self.placement_id == placement_id
+    }
+
+    pub fn has_placement_id(&self) -> bool {
+        self.placement_id.is_some()
     }
 
     pub fn top_left(&self) -> TextureCoordinate {

termwiz/src/surface/line.rs

@@ -405,6 +405,27 @@ impl Line {
     /// Similarly, when we assign a cell, we need to blank out those
     /// occluded successor cells.
     pub fn set_cell(&mut self, idx: usize, cell: Cell) -> &Cell {
+        self.set_cell_impl(idx, cell, false)
+    }
+
+    pub fn set_cell_clearing_image_placements(&mut self, idx: usize, cell: Cell) -> &Cell {
+        self.set_cell_impl(idx, cell, true)
+    }
+
+    fn raw_set_cell(&mut self, idx: usize, mut cell: Cell, clear: bool) {
+        if !clear {
+            if let Some(images) = self.cells[idx].attrs().images() {
+                for image in images {
+                    if image.has_placement_id() {
+                        cell.attrs_mut().attach_image(Box::new(image));
+                    }
+                }
+            }
+        }
+        self.cells[idx] = cell;
+    }
+
+    fn set_cell_impl(&mut self, idx: usize, cell: Cell, clear: bool) -> &Cell {
         let width = cell.width();
 
         // if the line isn't wide enough, pad it out with the default attributes.
@@ -428,10 +449,10 @@ impl Line {
         // For double-wide or wider chars, ensure that the cells that
         // are overlapped by this one are blanked out.
         for i in 1..=width.saturating_sub(1) {
-            self.cells[idx + i] = Cell::new(' ', cell.attrs().clone());
+            self.raw_set_cell(idx + i, Cell::new(' ', cell.attrs().clone()), clear);
         }
 
-        self.cells[idx] = cell;
+        self.raw_set_cell(idx, cell, clear);
         &self.cells[idx]
     }
 
@@ -529,7 +550,7 @@ impl Line {
     pub fn fill_range(&mut self, cols: Range<usize>, cell: &Cell) {
         for x in cols {
             // FIXME: we can skip the look-back for second and subsequent iterations
-            self.set_cell(x, cell.clone());
+            self.set_cell_impl(x, cell.clone(), true);
         }
         self.prune_trailing_blanks();
     }