Fix problem with tab size and subpixel positioning

Solve github issue https://github.com/franko/lite-xl/issues/109

data/core/docview.lua

@@ -370,7 +370,7 @@ function DocView:draw()
   self:draw_background(style.background)
 
   local font = self:get_font()
-  font:set_tab_width(font:get_width(" ") * config.indent_size)
+  font:set_tab_size(config.indent_size)
 
   local minline, maxline = self:get_visible_line_range()
   local lh = self:get_line_height()

src/api/renderer_font.c

@@ -48,10 +48,10 @@ static int f_load(lua_State *L) {
 }
 
 
-static int f_set_tab_width(lua_State *L) {
+static int f_set_tab_size(lua_State *L) {
   RenFont **self = luaL_checkudata(L, 1, API_TYPE_FONT);
   int n = luaL_checknumber(L, 2);
-  ren_set_font_tab_width(*self, n);
+  ren_set_font_tab_size(*self, n);
   return 0;
 }
 
@@ -98,7 +98,7 @@ static int f_get_height(lua_State *L) {
 static const luaL_Reg lib[] = {
   { "__gc",               f_gc                 },
   { "load",               f_load               },
-  { "set_tab_width",      f_set_tab_width      },
+  { "set_tab_size",       f_set_tab_size      },
   { "get_width",          f_get_width          },
   { "get_width_subpixel", f_get_width_subpixel },
   { "get_height",         f_get_height         },

src/rencache.c

@@ -20,7 +20,7 @@ typedef struct {
   RenFont *font;
   short int subpixel_scale;
   short int x_subpixel_offset;
-  int tab_width;
+  int tab_size;
   char text[0];
 } Command;
 
@@ -149,7 +149,7 @@ int rencache_draw_text(RenFont *font, const char *text, int x, int y, RenColor c
       cmd->rect = rect;
       cmd->subpixel_scale = (draw_subpixel ? subpixel_scale : 1);
       cmd->x_subpixel_offset = x - subpixel_scale * rect.x;
-      cmd->tab_width = ren_get_font_tab_width(font);
+      cmd->tab_size = ren_get_font_tab_size(font);
     }
   }
 
@@ -261,11 +261,11 @@ void rencache_end_frame(void) {
           ren_draw_rect(cmd->rect, cmd->color);
           break;
         case DRAW_TEXT:
-          ren_set_font_tab_width(cmd->font, cmd->tab_width);
+          ren_set_font_tab_size(cmd->font, cmd->tab_size);
           ren_draw_text(cmd->font, cmd->text, cmd->rect.x, cmd->rect.y, cmd->color);
           break;
         case DRAW_TEXT_SUBPIXEL:
-          ren_set_font_tab_width(cmd->font, cmd->tab_width);
+          ren_set_font_tab_size(cmd->font, cmd->tab_size);
           ren_draw_text_subpixel(cmd->font, cmd->text, cmd->subpixel_scale * cmd->rect.x + cmd->x_subpixel_offset, cmd->rect.y, cmd->color);
           break;
       }

src/renderer.c

@@ -23,6 +23,7 @@ struct RenFont {
   GlyphSet *sets[MAX_GLYPHSET];
   float size;
   int height;
+  int space_advance;
   FR_Renderer *renderer;
 };
 
@@ -145,6 +146,9 @@ RenFont* ren_load_font(const char *filename, float size, unsigned int renderer_f
   }
   font->height = FR_Get_Font_Height(font->renderer, size);
 
+  FR_Bitmap_Glyph_Metrics *gs = get_glyphset(font, ' ')->glyphs;
+  font->space_advance = gs[' '].xadvance;
+
   /* make tab and newline glyphs invisible */
   FR_Bitmap_Glyph_Metrics *g = get_glyphset(font, '\n')->glyphs;
   g['\t'].x1 = g['\t'].x0;
@@ -167,15 +171,15 @@ void ren_free_font(RenFont *font) {
 }
 
 
-void ren_set_font_tab_width(RenFont *font, int n) {
+void ren_set_font_tab_size(RenFont *font, int n) {
   GlyphSet *set = get_glyphset(font, '\t');
-  set->glyphs['\t'].xadvance = n;
+  set->glyphs['\t'].xadvance = font->space_advance * n;
 }
 
 
-int ren_get_font_tab_width(RenFont *font) {
+int ren_get_font_tab_size(RenFont *font) {
   GlyphSet *set = get_glyphset(font, '\t');
-  return set->glyphs['\t'].xadvance;
+  return set->glyphs['\t'].xadvance / font->space_advance;
 }
 
 

src/renderer.h

@@ -32,8 +32,8 @@ void ren_free_image(RenImage *image);
 
 RenFont* ren_load_font(const char *filename, float size, unsigned int renderer_flags);
 void ren_free_font(RenFont *font);
-void ren_set_font_tab_width(RenFont *font, int n);
-int ren_get_font_tab_width(RenFont *font);
+void ren_set_font_tab_size(RenFont *font, int n);
+int ren_get_font_tab_size(RenFont *font);
 int ren_get_font_width(RenFont *font, const char *text, int *subpixel_scale);
 int ren_get_font_height(RenFont *font);
 int ren_get_font_subpixel_scale(RenFont *font);