Clear some rendering artifacts from the rhs of the screen

These would get left behind after making the window wider
and then switching between alt screen and the main screen.

src/xwin.rs

@@ -895,6 +895,7 @@ impl<'a> TerminalWindow<'a> {
 
         // Break the line into clusters of cells with the same attributes
         let cell_clusters = line.cluster();
+        let mut last_cell_idx = 0;
         for cluster in cell_clusters {
             let attrs = &cluster.attrs;
             let is_highlited_hyperlink = match (&attrs.hyperlink, &current_highlight) {
@@ -985,6 +986,7 @@ impl<'a> TerminalWindow<'a> {
                         // smaller than the terminal.
                         break;
                     }
+                    last_cell_idx = cell_idx;
 
                     let selected = term::in_range(cell_idx, &selection);
                     let is_cursor = line_idx as i64 == cursor.y && cursor.x == cell_idx;
@@ -1084,6 +1086,23 @@ impl<'a> TerminalWindow<'a> {
             }
         }
 
+        // Clear any remaining cells to the right of the clusters we
+        // found above, otherwise we leave artifacts behind.  The easiest
+        // reproduction for the artifacts is to maximize the window and
+        // open a vim split horizontally.  Backgrounding vim would leave
+        // the right pane with its prior contents instead of showing the
+        // cleared lines from the shell in the main screen.
+        let bg_color = self.palette.background.to_linear_tuple_rgba();
+        let vert_idx = (last_cell_idx + 1) * VERTICES_PER_CELL;
+        let vert_slice = &mut vertices[vert_idx..];
+        for vert in vert_slice.iter_mut() {
+            vert.bg_color = bg_color;
+            vert.underline = U_NONE;
+            vert.tex = (0.0, 0.0);
+            vert.adjust = Default::default();
+            vert.has_color = 0.0;
+        }
+
         Ok(())
     }