Cannot update main when main is GridSpec

[MarcSkovMadsen]

ALL software version info

Panel MASTER branch

Description of expected behavior and the observed behavior

When the template main is a GridSpec I would expect to be able to update the GridSpec.

For example at awesome-panel.org I was trying to prepopulate a Template across pages to get a shared look and feel. But a few pages should look different and replace one or more of the prepopulated items.

Complete, minimal, self-contained example code that reproduces the issue

from panel.template import ReactTemplate
from panel.pane import Markdown

def test_grid_spec_replace():
    template=ReactTemplate()
    first = Markdown("I was here first")
    template.main[0,0]=first
    second=Markdown("But now I take over")
    template.main[0,0]=second
    return template

test_grid_spec_replace().servable()

Stack traceback and/or browser JavaScript console output

$ 2020-11-04 06:21:39,386 Starting Bokeh server version 2.2.3 (running on Tornado 6.0.4)
2020-11-04 06:21:39,387 User authentication hooks NOT provided (default user enabled)
2020-11-04 06:21:39,390 Bokeh app running at: http://localhost:5006/script
2020-11-04 06:21:39,391 Starting Bokeh server with process id: 21168
WARNING:param.GridSpec00005: Specified region overlaps with the following existing object(s) in the grid:

    (0, 0): Markdown(str)

The following shows a view of the grid (empty: 0, occupied: 1, overlapping: 2):

[[2 0 0 0 0 0 0 0 0 0 0 0]]
2020-11-04 06:21:47,318 Specified region overlaps with the following existing object(s) in the grid:

    (0, 0): Markdown(str)

The following shows a view of the grid (empty: 0, occupied: 1, overlapping: 2):

[[2 0 0 0 0 0 0 0 0 0 0 0]]
2020-11-04 06:21:47,636 WebSocket connection opened
2020-11-04 06:21:47,637 ServerConnection created

Screenshots or screencasts of the bug in action

Additional Context

I simply don't understand why the changes to the different parameters of a BasicTemplate needs to trigger complicated updates. I believe instead that there should be one function to update and render the template when needed. That would be much easier. I also think the current implementation has a performance cost because there will potentially be a lot of complicated updates that could have been simplified to one.

[MarcSkovMadsen]

This is a similar example. But this time an exception is raised

from panel.template import ReactTemplate
from panel.pane import Markdown
def test_grid_spec_replace2():
    template=ReactTemplate()
    first = Markdown("I was here first")
    template.main[0,0:12]=first
    second=Markdown("But now I take over")
    template.main[0,0:12]=second
    return template

test_grid_spec_replace2().servable()

$ python -m panel serve 'script.py' --dev
2020-11-04 06:42:11,350 Starting Bokeh server version 2.2.3 (running on Tornado 6.0.4)
2020-11-04 06:42:11,351 User authentication hooks NOT provided (default user enabled)
2020-11-04 06:42:11,355 Bokeh app running at: http://localhost:5006/script
2020-11-04 06:42:11,355 Starting Bokeh server with process id: 21168
2020-11-04 06:42:16,549 404 GET /home?template=react&theme=default (::1) 1.03ms
WARNING:param.GridSpec00005: Specified region overlaps with the following existing object(s) in the grid:

    (0, 0): Markdown(str)

The following shows a view of the grid (empty: 0, occupied: 1, overlapping: 2):

[[2 2 2 2 2 2 2 2 2 2 2 2]]
2020-11-04 06:42:22,961 Specified region overlaps with the following existing object(s) in the grid:

    (0, 0): Markdown(str)

The following shows a view of the grid (empty: 0, occupied: 1, overlapping: 2):

[[2 2 2 2 2 2 2 2 2 2 2 2]]
2020-11-04 06:42:22,964 Error running application handler <bokeh.application.handlers.script.ScriptHandler object at 0x00000240A6FB9488>: (0, 0, 1, 12)
File "grid.py", line 463, in __setitem__:
del self.objects[dkey] Traceback (most recent call last):
  File "C:\Users\masma\AppData\Local\Continuum\anaconda3\envs\panel_dev2\lib\site-packages\bokeh\application\handlers\code_runner.py", line 197, in run
    exec(self._code, module.__dict__)
  File "C:\repos\private\panel\script.py", line 11, in <module>
    test_grid_spec_replace2().servable()
  File "C:\repos\private\panel\script.py", line 8, in test_grid_spec_replace2
    template.main[0,0:12]=second
  File "C:\repos\private\panel\panel\layout\grid.py", line 463, in __setitem__
    del self.objects[dkey]
KeyError: (0, 0, 1, 12)

2020-11-04 06:42:23,045 WebSocket connection opened
2020-11-04 06:42:23,046 ServerConnection created