content scale change events should also impact viewport padding

This calculates the new padding pixel values and propogates those
changes to the renderer.

src/Surface.zig

@@ -139,6 +139,8 @@ const DerivedConfig = struct {
     mouse_interval: u64,
     macos_non_native_fullscreen: configpkg.NonNativeFullscreen,
     macos_option_as_alt: configpkg.OptionAsAlt,
+    window_padding_x: u32,
+    window_padding_y: u32,
 
     pub fn init(alloc_gpa: Allocator, config: *const configpkg.Config) !DerivedConfig {
         var arena = ArenaAllocator.init(alloc_gpa);
@@ -156,6 +158,8 @@ const DerivedConfig = struct {
             .mouse_interval = config.@"click-repeat-interval" * 1_000_000, // 500ms
             .macos_non_native_fullscreen = config.@"macos-non-native-fullscreen",
             .macos_option_as_alt = config.@"macos-option-as-alt",
+            .window_padding_x = config.@"window-padding-x",
+            .window_padding_y = config.@"window-padding-y",
 
             // Assignments happen sequentially so we have to do this last
             // so that the memory is captured from allocs above.
@@ -871,12 +875,19 @@ pub fn sizeCallback(self: *Surface, size: apprt.SurfaceSize) !void {
     // changed, so we just return.
     if (self.screen_size.equals(new_screen_size)) return;
 
+    try self.resize(new_screen_size);
+}
+
+fn resize(self: *Surface, size: renderer.ScreenSize) !void {
     // Save our screen size
-    self.screen_size = new_screen_size;
+    self.screen_size = size;
 
     // Mail the renderer so that it can update the GPU and re-render
     _ = self.renderer_thread.mailbox.push(.{
-        .screen_size = self.screen_size,
+        .resize = .{
+            .screen_size = self.screen_size,
+            .padding = self.padding,
+        },
     }, .{ .forever = {} });
     try self.queueRender();
 
@@ -1192,16 +1203,50 @@ pub fn scrollCallback(
     try self.queueRender();
 }
 
-pub fn contentScaleCallback(self: *Surface, content_scale: apprt.ContentScale) void {
+/// This is called when the content scale of the surface changes. The surface
+/// can then update any DPI-sensitive state.
+pub fn contentScaleCallback(self: *Surface, content_scale: apprt.ContentScale) !void {
+    // Calculate the new DPI
     const x_dpi = content_scale.x * font.face.default_dpi;
     const y_dpi = content_scale.y * font.face.default_dpi;
+
+    // Update our font size which is dependent on the DPI
     const size = size: {
         var size = self.font_size;
         size.xdpi = @intFromFloat(x_dpi);
         size.ydpi = @intFromFloat(y_dpi);
         break :size size;
     };
+
+    // If our DPI didn't actually change, save a lot of work by doing nothing.
+    if (size.xdpi == self.font_size.xdpi and size.ydpi == self.font_size.ydpi) {
+        return;
+    }
+
     self.setFontSize(size);
+
+    // Update our padding which is dependent on DPI.
+    self.padding = padding: {
+        const padding_x: u32 = padding_x: {
+            const padding_x: f32 = @floatFromInt(self.config.window_padding_x);
+            break :padding_x @intFromFloat(@floor(padding_x * x_dpi / 72));
+        };
+        const padding_y: u32 = padding_y: {
+            const padding_y: f32 = @floatFromInt(self.config.window_padding_y);
+            break :padding_y @intFromFloat(@floor(padding_y * y_dpi / 72));
+        };
+
+        break :padding .{
+            .top = padding_y,
+            .bottom = padding_y,
+            .right = padding_x,
+            .left = padding_x,
+        };
+    };
+
+    // Force a resize event because the change in padding will affect
+    // pixel-level changes to the renderer and viewport.
+    try self.resize(self.screen_size);
 }
 
 /// The type of action to report for a mouse event.

src/apprt/embedded.zig

@@ -350,7 +350,10 @@ pub const Surface = struct {
             .y = @floatCast(y),
         };
 
-        self.core_surface.contentScaleCallback(self.content_scale);
+        self.core_surface.contentScaleCallback(self.content_scale) catch |err| {
+            log.err("error in content scale callback err={}", .{err});
+            return;
+        };
     }
 
     pub fn updateSize(self: *Surface, width: u32, height: u32) void {

src/renderer/Metal.zig

@@ -1048,9 +1048,14 @@ pub fn changeConfig(self: *Metal, config: *DerivedConfig) !void {
 }
 
 /// Resize the screen.
-pub fn setScreenSize(self: *Metal, dim: renderer.ScreenSize) !void {
-    // Store our screen size
+pub fn setScreenSize(
+    self: *Metal,
+    dim: renderer.ScreenSize,
+    pad: renderer.Padding,
+) !void {
+    // Store our sizes
     self.screen_size = dim;
+    self.padding.explicit = pad;
 
     // Recalculate the rows/columns. This can't fail since we just set
     // the screen size above.

src/renderer/OpenGL.zig

@@ -1427,12 +1427,17 @@ pub fn changeConfig(self: *OpenGL, config: *DerivedConfig) !void {
 
 /// Set the screen size for rendering. This will update the projection
 /// used for the shader so that the scaling of the grid is correct.
-pub fn setScreenSize(self: *OpenGL, dim: renderer.ScreenSize) !void {
+pub fn setScreenSize(
+    self: *OpenGL,
+    dim: renderer.ScreenSize,
+    pad: renderer.Padding,
+) !void {
     if (single_threaded_draw) self.draw_mutex.lock();
     defer if (single_threaded_draw) self.draw_mutex.unlock();
 
     // Store our screen size
     self.screen_size = dim;
+    self.padding.explicit = pad;
 
     // Recalculate the rows/columns.
     const grid_size = self.gridSize(dim);

src/renderer/Thread.zig

@@ -252,8 +252,8 @@ fn drainMailbox(self: *Thread) !void {
                 try self.renderer.setFontSize(size);
             },
 
-            .screen_size => |size| {
-                try self.renderer.setScreenSize(size);
+            .resize => |v| {
+                try self.renderer.setScreenSize(v.screen_size, v.padding);
             },
 
             .change_config => |config| {

src/renderer/message.zig

@@ -20,8 +20,14 @@ pub const Message = union(enum) {
     /// the size changes.
     font_size: font.face.DesiredSize,
 
-    /// Change the screen size.
-    screen_size: renderer.ScreenSize,
+    /// Changes the screen size.
+    resize: struct {
+        /// The full screen (drawable) size. This does NOT include padding.
+        screen_size: renderer.ScreenSize,
+
+        /// The explicit padding values.
+        padding: renderer.Padding,
+    },
 
     /// The derived configuration to update the renderer with.
     change_config: struct {

src/renderer/size.zig

@@ -143,6 +143,14 @@ pub const Padding = struct {
             .left = self.left + other.left,
         };
     }
+
+    /// Equality test between two paddings.
+    pub fn eql(self: Padding, other: Padding) bool {
+        return self.top == other.top and
+            self.bottom == other.bottom and
+            self.right == other.right and
+            self.left == other.left;
+    }
 };
 
 test "Padding balanced on zero" {