config: clarify padding color default

src/config/Config.zig

@@ -712,10 +712,9 @@ keybind: Keybinds = .{},
 /// * The nearest row is a prompt row (requires shell integration). The
 ///   thinking here is that prompts often contain powerline glyphs that
 ///   do not look good extended.
+/// * The nearest row contains a perfect fit powerline character. These
+///   don't look good extended.
 ///
-/// The default value is "extend". This allows for smooth resizing of a
-/// terminal grid without having visible empty areas around the edge. The edge
-/// cells may appear slightly larger due to the extension.
 @"window-padding-color": WindowPaddingColor = .background,
 
 /// Synchronize rendering with the screen refresh rate. If true, this will

src/renderer/Metal.zig

@@ -1955,13 +1955,19 @@ pub fn setScreenSize(
     }).add(padding);
 
     var padding_extend = self.uniforms.padding_extend;
-    if (self.config.padding_color == .extend) {
-        // If padding extension is enabled, we extend left and right always.
+    switch (self.config.padding_color) {
+        .extend => {
+            // If padding extension is enabled, we extend left and right always
+            // because there is no downside to this. Up/down is dependent
+            // on some heuristics (see rebuildCells).
             padding_extend.left = true;
             padding_extend.right = true;
-    } else {
+        },
+
+        else => {
             // Otherwise, disable all padding extension.
             padding_extend = .{};
+        },
     }
 
     // Set the size of the drawable surface to the bounds
@@ -2138,9 +2144,12 @@ fn rebuildCells(
         self.cells.reset();
 
         // We also reset our padding extension depending on the screen type
-        if (self.config.padding_color == .extend) {
+        switch (self.config.padding_color) {
+            .background => {},
+            .extend => {
                 self.uniforms.padding_extend.up = screen_type == .alternate;
                 self.uniforms.padding_extend.down = screen_type == .alternate;
+            },
         }
     }