Fixes a bug where new BufPanes are not being inserted into the right array index.

When adding a new `BufPane` it is always being inserted last into `MainTab().Panes`.
This leads to a confusion when using the actions `PreviousSplit`, `NextSplit` as the previous/next split may not be the expected one.

How to reproduce:
- Launch micro and insert char "1"
- Open a new vsplit via the command `vsplit` and insert "2"
- Switch back to the left split (1) by using `PreviousSplit`
- Again open a new vsplit via command: `vsplit` and type char "3"
- Now switch between the 3 splits using `PreviousSplit`, `NextSplit`

Switching from most left split to the most right, the expected order would be 1, 3, 2 but actually is 1, 2, 3.

internal/action/bufpane.go

@@ -663,9 +663,13 @@ func (h *BufPane) DoRuneInsert(r rune) {
 func (h *BufPane) VSplitIndex(buf *buffer.Buffer, right bool) *BufPane {
 	e := NewBufPaneFromBuf(buf, h.tab)
 	e.splitID = MainTab().GetNode(h.splitID).VSplit(right)
-	MainTab().Panes = append(MainTab().Panes, e)
+	currentPaneIdx := MainTab().GetPane(h.splitID)
+	if right {
+		currentPaneIdx++
+	}
+	MainTab().AddPane(e, currentPaneIdx)
 	MainTab().Resize()
-	MainTab().SetActive(len(MainTab().Panes) - 1)
+	MainTab().SetActive(currentPaneIdx)
 	return e
 }
 
@@ -673,9 +677,13 @@ func (h *BufPane) VSplitIndex(buf *buffer.Buffer, right bool) *BufPane {
 func (h *BufPane) HSplitIndex(buf *buffer.Buffer, bottom bool) *BufPane {
 	e := NewBufPaneFromBuf(buf, h.tab)
 	e.splitID = MainTab().GetNode(h.splitID).HSplit(bottom)
-	MainTab().Panes = append(MainTab().Panes, e)
+	currentPaneIdx := MainTab().GetPane(h.splitID)
+	if bottom {
+		currentPaneIdx++
+	}
+	MainTab().AddPane(e, currentPaneIdx)
 	MainTab().Resize()
-	MainTab().SetActive(len(MainTab().Panes) - 1)
+	MainTab().SetActive(currentPaneIdx)
 	return e
 }
 

internal/action/tab.go

@@ -349,6 +349,16 @@ func (t *Tab) SetActive(i int) {
 	}
 }
 
+// AddPane adds a pane at a given index
+func (t *Tab) AddPane(pane Pane, i int) {
+	if len(t.Panes) == i {
+		t.Panes = append(t.Panes, pane)
+		return
+	}
+	t.Panes = append(t.Panes[:i+1], t.Panes[i:]...)
+	t.Panes[i] = pane
+}
+
 // GetPane returns the pane with the given split index
 func (t *Tab) GetPane(splitid uint64) int {
 	for i, p := range t.Panes {