mouse: Implement horizontal scroll. #3450

- Code from Vim source.
- Removed the check for 'guioptions'
- mouse_spec.lua: test <ScrollWheelLeft> and <ScrollWheelRight>
- Move horizontal scroll logic to mouse.c
- Remove 'gui_' from the function names
- Renamed variables to be more specific (as opposed to generic p, w).
- Marked some functions as `static`

src/nvim/mouse.c

@@ -14,6 +14,8 @@
 #include "nvim/misc1.h"
 #include "nvim/cursor.h"
 #include "nvim/buffer_defs.h"
+#include "nvim/memline.h"
+#include "nvim/charset.h"
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "mouse.c.generated.h"
@@ -503,3 +505,95 @@ void set_mouse_topline(win_T *wp)
   orig_topfill = wp->w_topfill;
 }
 
+///
+/// Return length of line "lnum" for horizontal scrolling.
+///
+static colnr_T scroll_line_len(linenr_T lnum)
+{
+  colnr_T col = 0;
+  char_u *line = ml_get(lnum);
+  if (*line != NUL) {
+    for (;;) {
+      int numchar = chartabsize(line, col);
+      mb_ptr_adv(line);
+      if (*line == NUL) {    // don't count the last character
+        break;
+      }
+      col += numchar;
+    }
+  }
+  return col;
+}
+
+///
+/// Find longest visible line number.
+///
+static linenr_T find_longest_lnum(void)
+{
+  linenr_T ret = 0;
+
+  // Calculate maximum for horizontal scrollbar.  Check for reasonable
+  // line numbers, topline and botline can be invalid when displaying is
+  // postponed.
+  if (curwin->w_topline <= curwin->w_cursor.lnum &&
+      curwin->w_botline > curwin->w_cursor.lnum &&
+      curwin->w_botline <= curbuf->b_ml.ml_line_count + 1) {
+    long max = 0;
+
+    // Use maximum of all visible lines.  Remember the lnum of the
+    // longest line, closest to the cursor line.  Used when scrolling
+    // below.
+    for (linenr_T lnum = curwin->w_topline; lnum < curwin->w_botline; lnum++) {
+      colnr_T len = scroll_line_len(lnum);
+      if (len > (colnr_T)max) {
+        max = len;
+        ret = lnum;
+      } else if (len == (colnr_T)max
+                 && abs((int)(lnum - curwin->w_cursor.lnum))
+                 < abs((int)(ret - curwin->w_cursor.lnum))) {
+        ret = lnum;
+      }
+    }
+  } else {
+    // Use cursor line only.
+    ret = curwin->w_cursor.lnum;
+  }
+
+  return ret;
+}
+
+///
+/// Do a horizontal scroll.  Return TRUE if the cursor moved, FALSE otherwise.
+///
+bool mouse_scroll_horiz(int dir)
+{
+  if (curwin->w_p_wrap) {
+      return false;
+  }
+
+  int step = 6;
+  if (mod_mask & (MOD_MASK_SHIFT | MOD_MASK_CTRL)) {
+      step = curwin->w_width;
+  }
+
+  int leftcol = curwin->w_leftcol + (dir == MSCR_RIGHT ? -step : +step);
+  if (leftcol < 0) {
+      leftcol = 0;
+  }
+
+  if (curwin->w_leftcol == leftcol) {
+      return false;
+  }
+
+  curwin->w_leftcol = (colnr_T)leftcol;
+
+  // When the line of the cursor is too short, move the cursor to the
+  // longest visible line.
+  if (!virtual_active()
+      && (colnr_T)leftcol > scroll_line_len(curwin->w_cursor.lnum)) {
+      curwin->w_cursor.lnum = find_longest_lnum();
+      curwin->w_cursor.col = 0;
+  }
+
+  return leftcol_changed();
+}