PR Summary

This PR makes changes to the Tk backend to prevent unnecessary recursion into the Tcl event loop, REPL hangs on figure close, and unnecessary draws on resize plus some extras.

Background

I have a project to explorestructured concurrency as applied to GUI design. Specifically, I have written a tkinter "app" that hoststrio inguest mode, and then uses callbacks exclusively to launch async functions, which themselves are eventually scheduled to run usingTk.after_idle.

Changes

Without going in to detail, it is easy to wind up in a situation whereFigureCanvasTkAgg.draw is called from an async function in such a way that it deadlocks the event loop. Fortunately, there are absolutely no legitimate reasons to invokeTk.update ortk.update_idletasks while an event loop is running, so this PR removes them entirely.

Other changes are less directly related to my primary motivation but became apparent as I was diagnosing the deadlock issue.

Resize events callFigureCanvasTkAgg.draw inFigureCanvasTkAgg.resize unnecessarily, asFigureCanvasBase.resize_event callsFigureCanvasTk.draw_idle. This PR removes that line, so resizes will be drawn eventually, without hogging the event loop.

NavigationToolbar2Tk has a special destroy method to delete it'stkinter.StringVar attribute namedmessage. It will be destroyed via Tk object tree (master=self) and GCd by Python (only referred to by other self attributes) without this, so this PR removes it outright.

FigureManagerTk has a destroy method that is completely ineffective and also fails to stop the Tk mainloop. This PR rewrites the method entirely usingprotocol("WM_DELETE_WINDOW",...) rather thanbind("<Destroy>",...). (Cherry picked and merged to master in#17802)

FigureCanvasTk is missing methodsstart_event_loop andstop_event_loop and so relies onFigureCanvasBase pure Python event loop implementations. This PR reimplements them following the example of the Qt and wx backends. However, I can't find any test code or Matplotlib API functions that actually exercise these methods.

Summary

These changes have minor or no user facing effect unless somewhereTk.after_idle schedules a callback that containsTk.update_idletasks. Writing this summary, it seems the fix toFigureManagerTk may actually the most user-noticeable (except most users are probably using Qt.) Not knowing whyTk.update_idletasks was sprinkled about in the first place, it is hard to say if this is backward-incompatable; it should be fine. A test to assert thatupdate orupdate_idletasks is never called when using the Tk backend would be appropriate, as well as a test of theFigureCanvasTk event loop methods, but I can't fathom how the backend tests are constructed, so some help would be appreciated.

PR Checklist

• Has Pytest style unit tests
• Code isFlake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way