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

1 files changed, 11 insertions, 0 deletions

diff --git a/src/fs/dirlist.rs b/src/fs/dirlist.rs
index b80e05b..e8dc90b 100644
--- a/src/fs/dirlist.rs
+++ b/src/fs/dirlist.rs
@@ -143,6 +143,17 @@ impl JoshutoDirList {
         self.get_curr_mut_(self.index?)
     }
 
+    /// For a given number of entries, visible in a UI, this method returns the index of the entry
+    /// with which the UI should start to list the entries.
+    ///
+    /// This method assures that the cursor is always in the viewport of the UI.
+    pub fn first_index_for_viewport(&self, viewport_height: usize) -> usize {
+        match self.index {
+            Some(index) => index / viewport_height as usize * viewport_height as usize,
+            None => 0,
+        }
+    }
+
     fn get_curr_mut_(&mut self, index: usize) -> Option<&mut JoshutoDirEntry> {
         if index < self.contents.len() {
             Some(&mut self.contents[index])