Fix selecting an entry by mouse click when show_borders=true (#77)

* encapsulate paging strategy

This commit introduces one central place where the “paging” (first entry
of a dir list shown in the UI) strategy is implemented.

This decreases the risk for copy-paste mistakes, makes it easier to
change it, and would make the implementation of a configurable paging
strategy easier.

* fix: correct new index on click for parent column

And a little refactoring of the code that handles the left click.

* fix: consider borders on left click

When selecting a dir entry with a left click of the mouse, the borders
were not considered, which led to

1. a faulty, constant y offset of 1 row
2. a faulty offset, which increased with each page scrolled down, due to
   a wrong calculation of the content height

2 files changed, 2 insertions, 2 deletions

diff --git a/src/ui/widgets/tui_dirlist.rs b/src/ui/widgets/tui_dirlist.rs
index 7d9f888..f66c9f0 100644
--- a/src/ui/widgets/tui_dirlist.rs
+++ b/src/ui/widgets/tui_dirlist.rs
@@ -36,7 +36,7 @@ impl<'a> Widget for TuiDirList<'a> {
         }
 
         let curr_index = self.dirlist.index.unwrap();
-        let skip_dist = curr_index / area.height as usize * area.height as usize;
+        let skip_dist = self.dirlist.first_index_for_viewport(area.height as usize);
 
         let drawing_width = area.width as usize;
 
diff --git a/src/ui/widgets/tui_dirlist_detailed.rs b/src/ui/widgets/tui_dirlist_detailed.rs
index 796ead4..2c0337d 100644
--- a/src/ui/widgets/tui_dirlist_detailed.rs
+++ b/src/ui/widgets/tui_dirlist_detailed.rs
@@ -41,7 +41,7 @@ impl<'a> Widget for TuiDirListDetailed<'a> {
         };
 
         let drawing_width = area.width as usize;
-        let skip_dist = curr_index / area.height as usize * area.height as usize;
+        let skip_dist = self.dirlist.first_index_for_viewport(area.height as usize);
 
         // draw every entry
         self.dirlist