Fix tab rendering to use dynamic tab width

Each tab is just wide enough to round to the nearest tab stop.

Refs #1243

helix-core/src/position.rs

@@ -83,7 +83,7 @@ pub fn visual_coords_at_pos(text: RopeSlice, pos: usize, tab_width: usize) -> Po
 
     for grapheme in RopeGraphemes::new(text.slice(line_start..pos)) {
         if grapheme == "\t" {
-            col += tab_width;
+            col += tab_width - (col % tab_width);
         } else {
             let grapheme = Cow::from(grapheme);
             col += grapheme_width(&grapheme);

helix-term/src/ui/editor.rs

@@ -378,7 +378,8 @@ impl EditorView {
 
                             let (grapheme, width) = if grapheme == "\t" {
                                 // make sure we display tab as appropriate amount of spaces
-                                (tab.as_str(), tab_width)
+                                let visual_tab_width = tab_width - (visual_x as usize % tab_width);
+                                (&tab[..visual_tab_width], visual_tab_width)
                             } else {
                                 // Cow will prevent allocations if span contained in a single slice
                                 // which should really be the majority case

helix-view/src/view.rs

@@ -243,18 +243,29 @@ impl View {
         let current_line = text.line(line_number);
 
         let target = (column - inner.x) as usize + self.offset.col;
-        let mut selected = 0;
+        let mut col = 0;
 
+        // TODO: extract this part as pos_at_visual_coords
         for grapheme in RopeGraphemes::new(current_line) {
-            if selected >= target {
+            if col >= target {
                 break;
             }
-            if grapheme == "\t" {
-                selected += tab_width;
+
+            let width = if grapheme == "\t" {
+                tab_width - (col % tab_width)
             } else {
-                let width = grapheme_width(&Cow::from(grapheme));
-                selected += width;
+                let grapheme = Cow::from(grapheme);
+                grapheme_width(&grapheme)
+            };
+
+            // If pos is in the middle of a wider grapheme (tab for example)
+            // return the starting offset.
+            if col + width >= target {
+                break;
             }
+
+            col += width;
+            // TODO: use byte pos that converts back to char pos?
             pos += grapheme.chars().count();
         }