fix(edit): backspace adds extra spaces for inline virtual text (#28005)

Problem:  Backspace adds extra spaces for inline virtual text.
Solution: Ignore virtual text and wrapping when backspacing.

src/nvim/edit.c

@@ -3673,23 +3673,6 @@ static void ins_del(void)
   AppendCharToRedobuff(K_DEL);
 }
 
-// Delete one character for ins_bs().
-static void ins_bs_one(colnr_T *vcolp)
-{
-  dec_cursor();
-  getvcol(curwin, &curwin->w_cursor, vcolp, NULL, NULL);
-  if (State & REPLACE_FLAG) {
-    // Don't delete characters before the insert point when in
-    // Replace mode
-    if (curwin->w_cursor.lnum != Insstart.lnum
-        || curwin->w_cursor.col >= Insstart.col) {
-      replace_do_bs(-1);
-    }
-  } else {
-    del_char(false);
-  }
-}
-
 /// Handle Backspace, delete-word and delete-line in Insert mode.
 ///
 /// @param          c                 character that was typed
@@ -3846,42 +3829,69 @@ static bool ins_bs(int c, int mode, int *inserted_space_p)
     // Handle deleting one 'shiftwidth' or 'softtabstop'.
     if (mode == BACKSPACE_CHAR
         && ((p_sta && in_indent)
-            || ((get_sts_value() != 0
-                 || tabstop_count(curbuf->b_p_vsts_array))
+            || ((get_sts_value() != 0 || tabstop_count(curbuf->b_p_vsts_array))
                 && curwin->w_cursor.col > 0
                 && (*(get_cursor_pos_ptr() - 1) == TAB
                     || (*(get_cursor_pos_ptr() - 1) == ' '
                         && (!*inserted_space_p || arrow_used)))))) {
-      colnr_T vcol;
-      colnr_T want_vcol;
-
       *inserted_space_p = false;
-      // Compute the virtual column where we want to be.  Since
-      // 'showbreak' may get in the way, need to get the last column of
-      // the previous character.
-      getvcol(curwin, &curwin->w_cursor, &vcol, NULL, NULL);
-      colnr_T start_vcol = vcol;
-      dec_cursor();
-      getvcol(curwin, &curwin->w_cursor, NULL, NULL, &want_vcol);
-      inc_cursor();
-      if (p_sta && in_indent) {
-        int ts = get_sw_value(curbuf);
-        want_vcol = (want_vcol / ts) * ts;
+
+      bool const use_ts = !curwin->w_p_list || curwin->w_p_lcs_chars.tab1;
+      char *const line = ml_get_buf(curbuf, curwin->w_cursor.lnum);
+      char *const end_ptr = line + curwin->w_cursor.col;
+
+      colnr_T vcol = 0;
+      colnr_T space_vcol = 0;
+      StrCharInfo sci = utf_ptr2StrCharInfo(line);
+      StrCharInfo space_sci = sci;
+      bool prev_space = false;
+      while (sci.ptr < end_ptr) {
+        bool cur_space = ascii_iswhite(sci.chr.value);
+        if (!prev_space && cur_space) {
+          space_sci = sci;
+          space_vcol = vcol;
+        }
+        vcol += charsize_nowrap(curbuf, use_ts, vcol, sci.chr.value);
+        sci = utfc_next(sci);
+        prev_space = cur_space;
+      }
+
+      colnr_T want_vcol = vcol - 1;
+      if (want_vcol <= 0) {
+        want_vcol = 0;
+      } else if (p_sta && in_indent) {
+        want_vcol = want_vcol - want_vcol % get_sw_value(curbuf);
       } else {
-        want_vcol = tabstop_start(want_vcol,
-                                  get_sts_value(),
-                                  curbuf->b_p_vsts_array);
+        want_vcol = tabstop_start(want_vcol, get_sts_value(), curbuf->b_p_vsts_array);
       }
 
-      // delete characters until we are at or before want_vcol
-      while (vcol > want_vcol && curwin->w_cursor.col > 0
-             && (cc = (uint8_t)(*(get_cursor_pos_ptr() - 1)), ascii_iswhite(cc))) {
-        ins_bs_one(&vcol);
+      while (true) {
+        int size = charsize_nowrap(curbuf, use_ts, space_vcol, space_sci.chr.value);
+        if (space_vcol + size > want_vcol) {
+          break;
+        }
+        space_vcol += size;
+        space_sci = utfc_next(space_sci);
+      }
+      colnr_T const want_col = (int)(space_sci.ptr - line);
+
+      // Delete characters until we are at or before want_col.
+      while (curwin->w_cursor.col > want_col) {
+        dec_cursor();
+        if (State & REPLACE_FLAG) {
+          // Don't delete characters before the insert point when in Replace mode.
+          if (curwin->w_cursor.lnum != Insstart.lnum
+              || curwin->w_cursor.col >= Insstart.col) {
+            replace_do_bs(-1);
+          }
+        } else {
+          del_char(false);
+        }
       }
 
-      // insert extra spaces until we are at want_vcol
-      while (vcol < want_vcol) {
-        // Remember the first char we inserted
+      // Insert extra spaces until we are at want_vcol.
+      for (; space_vcol < want_vcol; space_vcol++) {
+        // Remember the first char we inserted.
         if (curwin->w_cursor.lnum == Insstart_orig.lnum
             && curwin->w_cursor.col < Insstart_orig.col) {
           Insstart_orig.col = curwin->w_cursor.col;
@@ -3895,13 +3905,6 @@ static bool ins_bs(int c, int mode, int *inserted_space_p)
             replace_push(NUL);
           }
         }
-        getvcol(curwin, &curwin->w_cursor, &vcol, NULL, NULL);
-      }
-
-      // If we are now back where we started delete one character.  Can
-      // happen when using 'sts' and 'linebreak'.
-      if (vcol >= start_vcol) {
-        ins_bs_one(&vcol);
       }
     } else {
       // Delete up to starting point, start of line or previous word.

src/nvim/plines.c

@@ -375,6 +375,18 @@ CharSize charsize_fast(CharsizeArg *csarg, colnr_T const vcol, int32_t const cur
   return charsize_fast_impl(csarg->win, csarg->use_tabstop, vcol, cur_char);
 }
 
+/// Get the number of cells taken up on the screen at given virtual column.
+int charsize_nowrap(buf_T *buf, bool use_tabstop, colnr_T vcol, int32_t cur_char)
+{
+  if (cur_char == TAB && use_tabstop) {
+    return tabstop_padding(vcol, buf->b_p_ts, buf->b_p_vts_array);
+  } else if (cur_char < 0) {
+    return kInvalidByteCells;
+  } else {
+    return char2cells(cur_char);
+  }
+}
+
 /// Check that virtual column "vcol" is in the rightmost column of window "wp".
 ///
 /// @param  wp    window