Closes#1423
The bug was happening because when the Undo History window was
closed using the keyboard shortcut, the message kCloseMessage
was never being sent to the window. This is probably not the
only cause, but making UndoHistoryCommand close the window instead
of just hiding it solves the problem.
This is a work-in-progress to create a consistent API and finally
separate the whole Aseprite base/gfx/ui libs into a reusable C++ library.
Classes:
app::IFileItem, app::AppMenuItem, app::skin::SkinPart,
gfx::Rect, gfx::Border, she::FileDialog,
ui::IButtonIcon, ui::Graphics, ui::Overlay, ui::Widget,
ui::ScrollableViewDelegate, and UI events
Added member functions to undo::UndoHistory and
app::DocumentUndo to iterate all states of the undo history.
Also we’ve added app::DocumentUndoObserver to see when new states
are added in the undo history.
