[fractal] Fix stack overflow when switching to bold room

[Alexandre Franke <afranke src gnome org>]

commit b2a7a185829f7048c09ed4bf9bc2a8313fff6204
Author: Sonja Heinze <sonjaleaheinze gmail com>
Date:   Mon Feb 17 14:36:42 2020 +0100

    Fix stack overflow when switching to bold room
    
    Consider the following: the find_visible_index() recursion in the room
    history is initiated with only one message loaded in that room. That
    message is not detected as in sight.
    
    Then, before this commit, the recursion would never hit a base case and
    therefore lead to a stack overflow. This commit fixes that bug and makes
    sure that no similar bugs will be implemented in the future.

 fractal-gtk/src/widgets/room_history.rs | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)
---
diff --git a/fractal-gtk/src/widgets/room_history.rs b/fractal-gtk/src/widgets/room_history.rs
index 911c25b4..cc27416b 100644
--- a/fractal-gtk/src/widgets/room_history.rs
+++ b/fractal-gtk/src/widgets/room_history.rs
@@ -131,7 +131,7 @@ impl List {
         let sw = self.view.get_scrolled_window();
         let visible_index = match get_rel_position(&sw, &self.list[0]) {
             RelativePosition::InSight => Some(0),
-            _ => self.find_visible_index((1, len - 1)),
+            _ => self.find_visible_index((0, len - 1)),
         };
         if let Some(visible) = visible_index {
             indices.push(visible);
@@ -149,6 +149,11 @@ impl List {
     }
 
     fn find_visible_index(&self, range: (usize, usize)) -> Option<usize> {
+        /* Looks for a message widget in sight among all elements in rows.list.list of RoomHistory
+        whose corresponding index lies in the closed interval [range.0, range.1]. */
+        if range.0 > range.1 {
+            return None;
+        }
         let middle_index = (range.0 + range.1) / 2;
         let element = &self.list[middle_index];
         let scrolled_window = self.view.get_scrolled_window();