fix crash in help popup (closes #63)

Author: Stephan Dilly

CHANGELOG.md

@@ -6,6 +6,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+- fix crash help in small window size ([#63](https://github.com/extrawurst/gitui/issues/63))
+
 ## [0.2.5] - 2020-05-16
 ### Added
 - introduced proper changelog

src/ui/mod.rs

@@ -47,10 +47,10 @@ pub fn centered_rect_absolute(
     r: Rect,
 ) -> Rect {
     Rect::new(
-        (r.width - width) / 2,
-        (r.height - height) / 2,
-        width,
-        height,
+        (r.width.saturating_sub(width)) / 2,
+        (r.height.saturating_sub(height)) / 2,
+        width.min(r.width),
+        height.min(r.height),
     )
 }