editor: Rearrange float operations in layout (#9176)

Piotr Osiewicz created 2 years ago

We were seeing weird layouts with large files, where - starting with
some verylargelineindex - lines were rendered at weird y offsets. It
turned out that in some cases we're doing operations on Pixel values of
different magnitude, which then led to wrong results in calculations.
This commit addresses some of these problems, visible at glance when
working with large plaintext files. I *did not* dig into things like
inlay hints or diagnostics to see if they are subject to the same
potential precision loss.

Fixes #5371 

Release Notes:

- Fixed editor layout for large files, where the lines might have been
laid out with incorrect Y offset from the top.

Change summary

crates/editor/src/element.rs | 17 +++++++++--------
1 file changed, 9 insertions(+), 8 deletions(-)

Detailed changes

crates/editor/src/element.rs

@@ -856,7 +856,8 @@ impl EditorElement {
                     };
 
                     let x = cursor_character_x - scroll_pixel_position.x;
-                    let y = cursor_position.row() as f32 * line_height - scroll_pixel_position.y;
+                    let y = (cursor_position.row() as f32 - scroll_pixel_position.y / line_height)
+                        * line_height;
                     if selection.is_newest {
                         editor.pixel_position_of_newest_cursor = Some(point(
                             text_hitbox.origin.x + x + block_width / 2.,
@@ -1692,8 +1693,7 @@ impl EditorElement {
 
     fn paint_background(&self, layout: &EditorLayout, cx: &mut ElementContext) {
         cx.paint_layer(layout.hitbox.bounds, |cx| {
-            let scroll_top =
-                layout.position_map.snapshot.scroll_position().y * layout.position_map.line_height;
+            let scroll_top = layout.position_map.snapshot.scroll_position().y;
             let gutter_bg = cx.theme().colors().editor_gutter_background;
             cx.paint_quad(fill(layout.gutter_hitbox.bounds, gutter_bg));
             cx.paint_quad(fill(layout.text_hitbox.bounds, self.style.background));
@@ -1713,8 +1713,8 @@ impl EditorElement {
                         let origin = point(
                             layout.hitbox.origin.x,
                             layout.hitbox.origin.y
-                                + (layout.position_map.line_height * *start_row as f32)
-                                - scroll_top,
+                                + (*start_row as f32 - scroll_top)
+                                    * layout.position_map.line_height,
                         );
                         let size = size(
                             layout.hitbox.size.width,
@@ -1730,8 +1730,8 @@ impl EditorElement {
                         let origin = point(
                             layout.hitbox.origin.x,
                             layout.hitbox.origin.y
-                                + (layout.position_map.line_height * highlight_row_start as f32)
-                                - scroll_top,
+                                + (highlight_row_start as f32 - scroll_top)
+                                    * layout.position_map.line_height,
                         );
                         let size = size(
                             layout.hitbox.size.width,
@@ -2779,7 +2779,8 @@ impl LineWithInvisibles {
         cx: &mut ElementContext,
     ) {
         let line_height = layout.position_map.line_height;
-        let line_y = line_height * row as f32 - layout.position_map.scroll_pixel_position.y;
+        let line_y =
+            line_height * (row as f32 - layout.position_map.scroll_pixel_position.y / line_height);
 
         self.line
             .paint(