screen: defer Paint() call using an IdleEvent

This way, we avoid the assertion failure in
FileListPage::PaintListItem(), caused by FileBrowserPage::Reload()
resetting the FileList but not updating the ListWindow just yet; but
the mpdclient::GetConnection() could return an idle event which would
then trigger a synchronous repaint.

We avoid lots of undefined behavior by deferring all Paint() calls
with an IdleEvent, and maybe we save a few redundant Paint() calls.

Fixes https://bugs.debian.org/981152
Closes #87

NEWS

@@ -4,6 +4,7 @@ ncmpc 0.43 - not yet released
 * show "conductor" and "work" on song page
 * playlist editor (work in progress)
 * file page: handle mouse double clicks
+* fix crash bug
 * fix build failure on macOS
 * add azlyrics plugin
 

src/screen.cxx

@@ -222,7 +222,7 @@ ScreenManager::Update(struct mpdclient &c, const DelayedSeek &seek) noexcept
 	/* update the main window */
 	current_page->second->Update(c);
 
-	Paint();
+	SchedulePaint();
 }
 
 void

src/screen.hxx

@@ -26,6 +26,7 @@
 #include "StatusBar.hxx"
 #include "History.hxx"
 #include "Point.hxx"
+#include "event/IdleEvent.hxx"
 #include "util/Compiler.h"
 
 #include <curses.h>
@@ -44,7 +45,11 @@ class DelayedSeek;
 class EventLoop;
 
 class ScreenManager {
-	EventLoop &event_loop;
+	/**
+	 * This event defers Paint() calls until after the EventLoop
+	 * has handled all other events.
+	 */
+	IdleEvent paint_event;
 
 	struct Layout {
 		Size size;
@@ -105,7 +110,7 @@ public:
 	~ScreenManager() noexcept;
 
 	auto &GetEventLoop() const noexcept {
-		return event_loop;
+		return paint_event.GetEventLoop();
 	}
 
 	void Init(struct mpdclient *c) noexcept;
@@ -132,7 +137,6 @@ public:
 	void Swap(struct mpdclient &c, const struct mpd_song *song) noexcept;
 
 	void PaintTopWindow() noexcept;
-	void Paint() noexcept;
 
 	void Update(struct mpdclient &c, const DelayedSeek &seek) noexcept;
 	void OnCommand(struct mpdclient &c, DelayedSeek &seek, Command cmd);
@@ -144,6 +148,12 @@ public:
 
 private:
 	void NextMode(struct mpdclient &c, int offset) noexcept;
+
+	void SchedulePaint() noexcept {
+		paint_event.Schedule();
+	}
+
+	void Paint() noexcept;
 };
 
 #endif

src/screen_init.cxx

@@ -27,8 +27,8 @@
 static const unsigned SCREEN_MIN_COLS = 14;
 static const unsigned SCREEN_MIN_ROWS = 5;
 
-ScreenManager::ScreenManager(EventLoop &_event_loop) noexcept
-	:event_loop(_event_loop),
+ScreenManager::ScreenManager(EventLoop &event_loop) noexcept
+	:paint_event(event_loop, BIND_THIS_METHOD(Paint)),
 	 layout({std::max<unsigned>(COLS, SCREEN_MIN_COLS),
 		 std::max<unsigned>(LINES, SCREEN_MIN_ROWS)}),
 	 title_bar({layout.title_x, layout.title_y}, layout.size.width),
@@ -104,7 +104,7 @@ ScreenManager::OnResize() noexcept
 	curs_set(1);
 	curs_set(0);
 
-	Paint();
+	SchedulePaint();
 }
 
 void