Fix gutter overwriting other split pane

When we resize a split pane to a very small width, so that the gutter
does not fit in the pane, it overwrites the sibling split pane.

To fix it, clean up the calculation of gutter width, buffer width and
scrollbar width, so that they add up exactly to the window width, and
ensure that we don't draw the gutter beyond this calculated gutter
width (gutterOffset).

As a bonus, this also fixes the crash zyedidia#3052 (observed when resizing a
split pane to a very small width, if wordwrap is enabled), by ensuring
that bufWidth is never negative.

[*] By the gutter we mean of course gutter + diffgutter + ruler.

internal/display/bufwindow.go

@@ -146,12 +146,18 @@ func (w *BufWindow) updateDisplayInfo() {
 		w.gutterOffset += w.maxLineNumLength + 1
 	}
 
-	prevBufWidth := w.bufWidth
+	scrollbarWidth := 0
+	if w.Buf.Settings["scrollbar"].(bool) && w.Buf.LinesNum() > w.Height && w.Width > 0 {
+		scrollbarWidth = 1
+	}
 
-	w.bufWidth = w.Width - w.gutterOffset
-	if w.Buf.Settings["scrollbar"].(bool) && w.Buf.LinesNum() > w.Height {
-		w.bufWidth--
+	if w.gutterOffset > w.Width {
+		w.gutterOffset = w.Width
 	}
+	w.gutterOffset -= scrollbarWidth
+
+	prevBufWidth := w.bufWidth
+	w.bufWidth = w.Width - w.gutterOffset - scrollbarWidth
 
 	if w.bufWidth != prevBufWidth && w.Buf.Settings["softwrap"].(bool) {
 		for _, c := range w.Buf.GetCursors() {
@@ -277,13 +283,17 @@ func (w *BufWindow) drawGutter(vloc *buffer.Loc, bloc *buffer.Loc) {
 			break
 		}
 	}
-	screen.SetContent(w.X+vloc.X, w.Y+vloc.Y, char, nil, s)
-	vloc.X++
-	screen.SetContent(w.X+vloc.X, w.Y+vloc.Y, char, nil, s)
-	vloc.X++
+	for i := 0; i < 2 && vloc.X < w.gutterOffset; i++ {
+		screen.SetContent(w.X+vloc.X, w.Y+vloc.Y, char, nil, s)
+		vloc.X++
+	}
 }
 
 func (w *BufWindow) drawDiffGutter(backgroundStyle tcell.Style, softwrapped bool, vloc *buffer.Loc, bloc *buffer.Loc) {
+	if vloc.X >= w.gutterOffset {
+		return
+	}
+
 	symbol := ' '
 	styleName := ""
 
@@ -319,26 +329,28 @@ func (w *BufWindow) drawLineNum(lineNumStyle tcell.Style, softwrapped bool, vloc
 	} else {
 		lineInt = bloc.Y - cursorLine
 	}
-	lineNum := strconv.Itoa(util.Abs(lineInt))
+	lineNum := []rune(strconv.Itoa(util.Abs(lineInt)))
 
 	// Write the spaces before the line number if necessary
-	for i := 0; i < w.maxLineNumLength-len(lineNum); i++ {
+	for i := 0; i < w.maxLineNumLength-len(lineNum) && vloc.X < w.gutterOffset; i++ {
 		screen.SetContent(w.X+vloc.X, w.Y+vloc.Y, ' ', nil, lineNumStyle)
 		vloc.X++
 	}
 	// Write the actual line number
-	for _, ch := range lineNum {
+	for i := 0; i < len(lineNum) && vloc.X < w.gutterOffset; i++ {
 		if softwrapped {
 			screen.SetContent(w.X+vloc.X, w.Y+vloc.Y, ' ', nil, lineNumStyle)
 		} else {
-			screen.SetContent(w.X+vloc.X, w.Y+vloc.Y, ch, nil, lineNumStyle)
+			screen.SetContent(w.X+vloc.X, w.Y+vloc.Y, lineNum[i], nil, lineNumStyle)
 		}
 		vloc.X++
 	}
 
 	// Write the extra space
-	screen.SetContent(w.X+vloc.X, w.Y+vloc.Y, ' ', nil, lineNumStyle)
-	vloc.X++
+	if vloc.X < w.gutterOffset {
+		screen.SetContent(w.X+vloc.X, w.Y+vloc.Y, ' ', nil, lineNumStyle)
+		vloc.X++
+	}
 }
 
 // getStyle returns the highlight style for the given character position