New function to get the transform that will be used for the stacking context

This fixes @notriddle's MR when rebased on top of the changes to have
a ValidTransform instead of just Transform in DrawingCtx.

The text code needs to know the actual transform that will be used on
the target surface, which could be the temporary surface for
compositing an isolated stacking context, or the "normal" surface
without isolation.

This is the transform that is passed to the draw_fn callback in with_discrete_layer().

Part-of: <https://gitlab.gnome.org/GNOME/librsvg/-/merge_requests/816>

2 files changed, 22 insertions, 8 deletions

diff --git a/src/drawing_ctx.rs b/src/drawing_ctx.rs
index c9cc9232..d6632888 100644
--- a/src/drawing_ctx.rs
+++ b/src/drawing_ctx.rs
@@ -358,8 +358,26 @@ impl DrawingCtx {
         *self.initial_viewport.vbox
     }
 
-    pub fn toplevel_transform(&self) -> Transform {
-        self.initial_viewport.transform.clone()
+    /// Gets the transform that will be used on the target surface,
+    /// whether using an isolated stacking context or not.
+    ///
+    /// This is only used in the text code, and we should probably try
+    /// to remove it.
+    pub fn get_transform_for_stacking_ctx(
+        &self,
+        stacking_ctx: &StackingContext,
+    ) -> Result<ValidTransform, RenderingError> {
+        if stacking_ctx.should_isolate() {
+            let affines = CompositingAffines::new(
+                *self.get_transform(),
+                self.initial_viewport.transform,
+                self.cr_stack.borrow().len(),
+            );
+
+            Ok(ValidTransform::try_from(affines.for_temporary_surface)?)
+        } else {
+            Ok(self.get_transform())
+        }
     }
 
     pub fn is_measuring(&self) -> bool {
diff --git a/src/text.rs b/src/text.rs
index ca3598cf..77e3fb60 100644
--- a/src/text.rs
+++ b/src/text.rs
@@ -784,11 +784,7 @@ impl ElementTrait for Text {
         );
 
         let layout_text = {
-            let transform = if stacking_ctx.should_isolate() {
-                draw_ctx.toplevel_transform()
-            } else {
-                draw_ctx.get_transform().into::<Transform>()
-            };
+            let transform = draw_ctx.get_transform_for_stacking_ctx(&stacking_ctx)?;
 
             let layout_context = LayoutContext {
                 writing_mode: values.writing_mode(),
@@ -829,7 +825,7 @@ impl ElementTrait for Text {
                 }
             }
 
-            let empty_bbox = BoundingBox::new().with_transform(transform);
+            let empty_bbox = BoundingBox::new().with_transform(*transform);
 
             let text_bbox = layout_spans.iter().fold(empty_bbox, |mut bbox, span| {
                 if let Some(ref span_bbox) = span.bbox {