Enhance Tabbar Click

[cutelisp]

Currently, in the tab bar, each tab is separated by a gap of 4 cells, usually.
When the gap is left-clicked, the tab on the right side of the gap is selected.
This PR makes the tab nearest to the click the selected one.

[dmaluka]

This causes a side effect: when clicking on the empty space after the last tab, it activates the last tab if the click is 1 or 2 (but not 3 or more) spaces after the last tab name.

Also a note unrelated to this PR: it seems pointless to check vloc.Y == w.Y at each iteration of the loop, we can just check it at the beginning.

So, how about such change:

diff --git a/internal/display/tabwindow.go b/internal/display/tabwindow.go
index 423b966b..96aead73 100644
--- a/internal/display/tabwindow.go
+++ b/internal/display/tabwindow.go
@@ -29,12 +29,19 @@ func (w *TabWindow) Resize(width, height int) {
 }
 
 func (w *TabWindow) LocFromVisual(vloc buffer.Loc) int {
+	if vloc.Y != w.Y {
+		return -1
+	}
+
 	x := -w.hscroll
 
 	for i, n := range w.Names {
 		x++
 		s := util.CharacterCountInString(n)
-		if vloc.Y == w.Y && vloc.X < x+s+2 {
+		if vloc.X < x+s {
+			return i
+		}
+		if i < len(w.Names)-1 && vloc.X < x+s+2 {
 			return i
 		}
 		x += s

[JoeKar]

This causes a side effect: when clicking on the empty space after the last tab, it activates the last tab if the click is 1 or 2 (but not 3 or more) spaces after the last tab name.

Here I'm quite undecided.
When we have 3 tabs (a, b & c) open then we can activate:

• a+2
• 2+b+2
• 2+c

Clicking on the left side of a is limited by the boundary of the window, but the right side of c most probably isn't.
Now we have a active and click slightly of the right side after c then c it isn't activated.
We could activate c with the whole remaining space (by adding a check for i == len(w.Names)-1 at the end of the loop and return i too). What do you think?

While we are here already, then we should also fix the use case of wide runes in tab names (also note in #3516) by exchanging all util.CharacterCountInString() to runewidth.StringWidth() to proper address the x position and scroll width.