cef hangs while resizing in certain conditions

[MarkusFF]

Specification

• pywebview version: 3.2
• platform / version: Win7 Pro SP1
• cefpython3 version 66.0

Description

Observed reproducible hang during use of cef gui, if triggering some kind of update while resizing the window. Minimally this code usually (but not 100% of the time) hangs on my machine:

import webview
window=webview.create_window('Hello world', html='<h1>hello</h1>')
def reload_and_resize():
    window.evaluate_js("location.reload()")
    window.resize(1000, 420)
webview.start(reload_and_resize, gui="cef")

or while manually resizing something more like this:

import webview,time
window=webview.create_window('Hello world', html='<h1>hello</h1>')
def keep_reloading():
    while True:
        time.sleep(1)
        window.evaluate_js("location.reload()")
webview.start(keep_reloading, gui="cef")

This may be a cefpython issue.

[MarkusFF]

Could be this: cztomczak/cefpython#464

[r0x0r]

I have reproduced it and it looks like a race condition issue to me. At least adding a debug print statement somewhere in an execution chain seem to eliminate the problem. I will look into it.

[github-actions]

This issue is stale because it has been open 30 days with no activity. Remove stale label or comment or this will be closed in 5 days

[ikhz]

I have reproduced it and it looks like a race condition issue to me. At least adding a debug print statement somewhere in an execution chain seem to eliminate the problem. I will look into it.

Is there any news?

[r0x0r]

I gave it a try, but did not make any progress. The problem is that as soon as I add any debugging statements, the problem is gone. For example

This hangs

import webview
window=webview.create_window('Hello world', html='<h1>hello</h1>')
def reload_and_resize():
    window.evaluate_js("location.reload()")
    window.resize(1000, 420)
webview.start(reload_and_resize, gui="cef")

This does not

import webview
window=webview.create_window('Hello world', html='<h1>hello</h1>')
def reload_and_resize():
    window.evaluate_js("location.reload()")
    print('w00t')
    window.resize(1000, 420)
webview.start(reload_and_resize, gui="cef")

Similarly adding print statements elsewhere in the CEF code eliminates the problem. Breakpoints set in sub threads are not invoked either. A dumb solution would be add a short sleep noop.

The second example in the original post did not result in hanging on my machine.

[ikhz]

This problem is fixed in cefpython repo in dev branch (I hope so), but I had no luck in building it on Win 10 so I made changes to cef.py here in pywebview

#add inner window hwnd to Browser class
class Browser:
    def __init__(self, window, handle, browser):
        ...
        self.inner_hwnd = self.browser.GetWindowHandle()
        ...

# resize through win api
@_cef_call
def resize(width, height, uid):
    instance = instances[uid]
    win32gui.SetWindowPos(instance.inner_hwnd, 0, 0, 0, width-16, height-38, win32con.SWP_NOZORDER | win32con.SWP_NOMOVE | win32con.SWP_NOACTIVATE)
    instance.browser.NotifyMoveOrResizeStarted()

width-16, height-38 is a temporary solution to not let inner window become bigger than outer and content on the sides not visible

[r0x0r]

Have you tested if this fixes the problem?

[ikhz]

Have you tested if this fixes the problem?

Not a single hang after this patch, tried to resize window with mouse in different ways for a long time.
Your sample that hangs also not hanging

[r0x0r]

Nice one. Would you mind creating a PR?

[ikhz]

Ok I'll try, but this is temporary solution until cefpython release 66.1 version (after that Browser.SetBounds method will be available on Win and we should use it to resize window)

[ikhz]

#504

[r0x0r]

@MarkusFF Can you verify if @ikhmyz PR fixes this problem for you?