fix: keep tab-bar margin transitions in sync with hide-if-single

resize_top_or_bottom_line compared the new padding against
dimension.margin, but apply_taffy_layout zeroes dimension.margin on
every run. Once that has happened the "previous" margin reads as zero;
when num_tabs drops to 1 and hide_if_single makes padding_y_top zero
too, the comparison short-circuits and update_scaled_margin is never
called. Compare against the grid's scaled_margin (divided by scale)
instead, since that holds the authoritative current margin.

Without this, closing the second tab with hide-if-single enabled left
the tab-bar strip reserved at the top and terminal content drew below
it instead of filling the freed area.

Author: shiena

frontends/rioterm/src/screen/mod.rs

@@ -1580,8 +1580,6 @@ impl Screen<'_> {
     }
 
     pub fn resize_top_or_bottom_line(&mut self, num_tabs: usize) {
-        let layout = self.context_manager.current().dimension;
-        let previous_margin = layout.margin;
         let padding_y_top = padding_top_from_config(
             &self.renderer.navigation,
             self.renderer.margin.top,
@@ -1590,8 +1588,16 @@ impl Screen<'_> {
         );
         let padding_y_bottom = self.renderer.margin.bottom;
 
-        if previous_margin.top != padding_y_top
-            || previous_margin.bottom != padding_y_bottom
+        // Compare against the grid's scaled_margin (the actual current margin)
+        // rather than dimension.margin, which apply_taffy_layout zeroes out and
+        // therefore cannot tell us whether the top-bar margin changed.
+        let scale = self.sugarloaf.scale_factor();
+        let current_scaled_margin = self.context_manager.current_grid().scaled_margin;
+        let previous_margin_top = current_scaled_margin.top / scale;
+        let previous_margin_bottom = current_scaled_margin.bottom / scale;
+
+        if previous_margin_top != padding_y_top
+            || previous_margin_bottom != padding_y_bottom
         {
             if let Some(layout) = self
                 .sugarloaf
@@ -1601,7 +1607,6 @@ impl Screen<'_> {
                 s.font_size = layout.font_size;
                 s.line_height = layout.line_height;
 
-                let scale = self.sugarloaf.scale_factor();
                 let d = self.context_manager.current_grid_mut();
                 d.update_scaled_margin(Margin::new(
                     padding_y_top * scale,