Adjust StyleBoxFlat antialiasing to account for 2D stretch scale

This prevents the antialiasing feather from becoming too wide
at viewport sizes higher than the default, which can lead to blurry visuals.

This is adjusted to account for the 2D scale factor returned on the root
Window, so it takes both the `canvas_items` scaling and
`content_scale_factor` into account.

scene/resources/style_box_flat.cpp

@@ -30,6 +30,8 @@
 
 #include "style_box_flat.h"
 
+#include "scene/main/scene_tree.h"
+#include "scene/main/window.h"
 #include "servers/rendering_server.h"
 
 float StyleBoxFlat::get_style_margin(Side p_side) const {
@@ -415,10 +417,28 @@ void StyleBoxFlat::draw(RID p_canvas_item, const Rect2 &p_rect) const {
 	Rect2 infill_rect = style_rect.grow_individual(-adapted_border[SIDE_LEFT], -adapted_border[SIDE_TOP], -adapted_border[SIDE_RIGHT], -adapted_border[SIDE_BOTTOM]);
 
 	Rect2 border_style_rect = style_rect;
+
+	float aa_size_scaled = 1.0f;
+	if (aa_on) {
+		float scale_factor = 1.0f;
+		const SceneTree *tree = Object::cast_to<SceneTree>(OS::get_singleton()->get_main_loop());
+		if (tree) {
+			const Window *window = tree->get_root();
+			const Vector2 stretch_scale = window->get_stretch_transform().get_scale();
+			scale_factor = MIN(stretch_scale.x, stretch_scale.y);
+		}
+
+		// Adjust AA feather size to account for the 2D scale factor, so that
+		// antialiasing doesn't become blurry at viewport resolutions higher
+		// than the default when using the `canvas_items` stretch mode
+		// (or when using `content_scale_factor` values different than `1.0`).
+		aa_size_scaled = aa_size / scale_factor;
+	}
+
 	if (aa_on) {
 		for (int i = 0; i < 4; i++) {
 			if (border_width[i] > 0) {
-				border_style_rect = border_style_rect.grow_side((Side)i, -aa_size);
+				border_style_rect = border_style_rect.grow_side((Side)i, -aa_size_scaled);
 			}
 		}
 	}
@@ -464,26 +484,27 @@ void StyleBoxFlat::draw(RID p_canvas_item, const Rect2 &p_rect) const {
 		real_t aa_border_width_half[4];
 		real_t aa_fill_width[4];
 		real_t aa_fill_width_half[4];
+
 		if (draw_border) {
 			for (int i = 0; i < 4; i++) {
 				if (border_width[i] > 0) {
-					aa_border_width[i] = aa_size;
-					aa_border_width_half[i] = aa_size / 2;
+					aa_border_width[i] = aa_size_scaled;
+					aa_border_width_half[i] = aa_size_scaled * 0.5;
 					aa_fill_width[i] = 0;
 					aa_fill_width_half[i] = 0;
 				} else {
 					aa_border_width[i] = 0;
 					aa_border_width_half[i] = 0;
-					aa_fill_width[i] = aa_size;
-					aa_fill_width_half[i] = aa_size / 2;
+					aa_fill_width[i] = aa_size_scaled;
+					aa_fill_width_half[i] = aa_size_scaled * 0.5;
 				}
 			}
 		} else {
 			for (int i = 0; i < 4; i++) {
 				aa_border_width[i] = 0;
 				aa_border_width_half[i] = 0;
-				aa_fill_width[i] = aa_size;
-				aa_fill_width_half[i] = aa_size / 2;
+				aa_fill_width[i] = aa_size_scaled;
+				aa_fill_width_half[i] = aa_size_scaled * 0.5;
 			}
 		}
 
@@ -536,7 +557,7 @@ void StyleBoxFlat::draw(RID p_canvas_item, const Rect2 &p_rect) const {
 	}
 
 	// Compute UV coordinates.
-	Rect2 uv_rect = style_rect.grow(aa_on ? aa_size : 0);
+	Rect2 uv_rect = style_rect.grow(aa_on ? aa_size_scaled : 0);
 	uvs.resize(verts.size());
 	for (int i = 0; i < verts.size(); i++) {
 		uvs.write[i].x = (verts[i].x - uv_rect.position.x) / uv_rect.size.width;