Pune/India: Rustic way of handling the mark panes disappearance

Interesting: before that fix, the GUI was in an invalid state,
as the mark pane disappeared without setting the focus back to the
main window.
Thanks to everything being encoded in the typesystem and no assumptions
made, the program wouldn't crash when buttons are pressed, which would
otherwise be attempted to dispatch to the now gone mark pane.

Instead one could press 'q' or tab, as fortunately, the pane
handling is always done before any of the panes get a chance.

src/interactive/app/eventloop.rs

@@ -102,7 +102,7 @@ impl TerminalApp {
             }
 
             match self.state.focussed {
-                FocussedPane::Mark => self.window.mark_pane.as_mut().expect("mark pane").key(key),
+                FocussedPane::Mark => self.dispatch_to_mark_pane(key),
                 FocussedPane::Help => {
                     self.window.help_pane.as_mut().expect("help pane").key(key);
                 }

src/interactive/app/handlers.rs

@@ -1,11 +1,12 @@
 use crate::interactive::{
-    app::{FocussedPane, TerminalApp},
+    app::{FocussedPane::*, TerminalApp},
     sorted_entries,
     widgets::{HelpPane, MarkPane},
 };
 use dua::path_of;
 use itertools::Itertools;
 use petgraph::Direction;
+use termion::event::Key;
 
 pub enum CursorDirection {
     PageDown,
@@ -28,7 +29,6 @@ impl CursorDirection {
 
 impl TerminalApp {
     pub fn cycle_focus(&mut self) {
-        use FocussedPane::*;
         if let Some(p) = self.window.mark_pane.as_mut() {
             p.set_focus(false)
         };
@@ -53,7 +53,6 @@ impl TerminalApp {
     }
 
     pub fn toggle_help_pane(&mut self) {
-        use FocussedPane::*;
         self.state.focussed = match self.state.focussed {
             Main | Mark => {
                 self.window.help_pane = Some(HelpPane::default());
@@ -145,4 +144,11 @@ impl TerminalApp {
             self.change_entry_selection(CursorDirection::Down)
         }
     }
+
+    pub fn dispatch_to_mark_pane(&mut self, key: Key) -> () {
+        self.window.mark_pane = self.window.mark_pane.take().and_then(|p| p.key(key));
+        if self.window.mark_pane.is_none() {
+            self.state.focussed = Main;
+        }
+    }
 }

src/interactive/widgets/mark.rs

@@ -82,18 +82,19 @@ impl MarkPane {
     pub fn marked(&self) -> &EntryMarkMap {
         &self.marked
     }
-    pub fn key(&mut self, key: Key) {
+    pub fn key(mut self, key: Key) -> Option<Self> {
         match key {
-            Char('d') | Char(' ') => self.remove_selected_and_move_down(),
+            Char('d') | Char(' ') => return self.remove_selected_and_move_down(),
             Ctrl('u') | PageUp => self.change_selection(CursorDirection::PageUp),
             Char('k') | Up => self.change_selection(CursorDirection::Up),
             Char('j') | Down => self.change_selection(CursorDirection::Down),
             Ctrl('d') | PageDown => self.change_selection(CursorDirection::PageDown),
             _ => {}
         };
+        Some(self)
     }
 
-    fn remove_selected_and_move_down(&mut self) {
+    fn remove_selected_and_move_down(mut self) -> Option<Self> {
         if let Some(selected) = self.selected {
             let (idx, se_len) = {
                 let sorted_entries: Vec<_> = self
@@ -108,10 +109,14 @@ impl MarkPane {
             };
             if let Some(idx) = idx {
                 self.marked.remove(&idx);
-                if selected == se_len.saturating_sub(1) {
-                    self.selected = selected.checked_sub(1);
+                if se_len.saturating_sub(1) == 0 {
+                    return None;
                 }
+                self.selected = selected.checked_sub(1);
             }
+            Some(self)
+        } else {
+            Some(self)
         }
     }
 
@@ -208,7 +213,7 @@ impl MarkPane {
         let inner_area = block.inner(area);
         block.draw(area, buf);
 
-        let list_area = if self.has_focus && !self.marked.is_empty() {
+        let list_area = if self.has_focus {
             let (help_line_area, list_area) = {
                 let regions = Layout::default()
                     .direction(Direction::Vertical)