Item 7 from #33610:
Change fixed # of lines to variable # of lines as needed, up to limit.  About 15 is limit for 4-space indents in 80 char lines. 

* Change default number of lines to be 15 in config.
* Label in config dialog.
* Initialize context to be one blank line to show it's turned on.
* Remove padding of blank lines.
* Modify tests to show variable number of lines in context text instead of lines scrolling out of view.

Tip: For manual tests, help.py works really well in viewing 15 lines.

2 quick thoughts.

1. I am not sure we can/should change the default, as it interacts with the recorded user changes for past versions.  The default is that we never change defaults.  (Having one user config for all Python/IDLE versions is a curse as well as a blessing.  I anticipate having to do something different, sometime.

2. I was thinking for '<top level>' instead of a blank line for when CC is on but there is no real context.  I think we should try both and see what seems better in use while moving down through a file.

I know we usually don't change defaults, but I had tested this one and I think it works.  After I changed it to 15 lines in master, I opened the installed version of IDLE 2.7.  Although it showed 15 fixed lines, it made sense since that was the number of lines in the 3.8 version.

I figured a typical user would be using whichever version of IDLE comes with their current Python.  So, at some point the OS would upgrade it and they would be moving from a version with fixed lines to a version with variable lines (for Python 3).  If they had previously changed their settings, it would still use whatever they had changed it to as the limit, but now the context area would grow and shrink (so they would know the change exists).  If they hadn't customized their settings, it would just change from 3 to 15.

If the user switches between two versions (like when I tested with 2.7), then I think seeing the changes almost makes more sense to them.  They can see it at 15 in the new version and watch it grow and shrink.  Then it the 2.7 version, it looks the same, but without the expansion/contraction.  It's a demonstration of what makes the variable sizing nice to have.  Although it may lead to someone wanting it in 2.7.  :-)

Anyway, that was my thinking process.  When you try it out, you may not agree, but it made sense to me when I saw what was happening.

Note: one time this didn't quite work was when I set the line limit to 90.  3.8 worked great -- just like it had with 15 (that is, it never ate the whole editor window).  But, under 2.7, it ate the whole window and without the `reload` in codecontext, changing the config didn't help until I restarted IDLE.

Unlinked duplicate PR entry.  I have no idea how that could happen.

After testing the patch as a user, I would like 3 user-visible changes.

1. Only display a blank line when there are no context lines.

2. Replace the 'lines' option with a 'maxlines' option.  There is no need to worry about, or explain to users (see 3.) the possible complications of reusing 'lines' with a different meaning.

3. Add a 'maxlines' entry to the General tab help page.  I will work on this either first or second.

I will post other thoughts on the master issue, #33610.

I pushed a commit for these three items.

On Windows, I observe the following behaviors.

1. When Code Context is enabled on a windows still at its initial height, context lines are added to code lines to increase the window height.

2. When the window height is changed with a mouse, that becomes the fixed height, which is partitioned between context and code lines.  In other words, context lines are taken from and given back to code lines.

3. When height is maximized either with ZoomHeight or WindowMaximize, the behavior is as with 2.  When height is un-maximized, behavior reverts to what it was, either 1 or 2.

What do you see on Linux?

I see similar behavior on Linux, except after maximizing and then minimizing, it seems to stay with option 2.

New changeset 29996a1c4e8bd6dde6adce2b44d11a0982a47a3a by Terry Jan Reedy (Cheryl Sabella) in branch 'master':
bpo-33642: IDLE: Use variable number of lines in CodeContext. (GH-7106)
https://github.com/python/cpython/commit/29996a1c4e8bd6dde6adce2b44d11a0982a47a3a

New changeset 0800b6c17a829ecb554d93cba8c97f92b15be52a by Miss Islington (bot) in branch '3.7':
bpo-33642: IDLE: Use variable number of lines in CodeContext. (GH-7106)
https://github.com/python/cpython/commit/0800b6c17a829ecb554d93cba8c97f92b15be52a

New changeset a42fe3c67ca86ac08dc6f47af27eed089d35fdd0 by Miss Islington (bot) in branch '3.6':
bpo-33642: IDLE: Use variable number of lines in CodeContext. (GH-7106)
https://github.com/python/cpython/commit/a42fe3c67ca86ac08dc6f47af27eed089d35fdd0

I am leaving this open until the new behavior is documented, even though it might be part of another issue that includes other context changes.

For this issue, I will leave what happens when unzooming unspecific.

Summary for users: Previously, Code Context displayed a fixed number of lines.  Often, many were blank.  Hence the default *numlines* to display was small, 3, to avoid having too many blank lines.  However, 3 is often too small to display the complete context.

Now, Code Context initially displays a single blank line for 'no context', to show that the feature is active.  When the top visible line is part of an indented body, the context used as many non-blank lines as needed, up to a maximum of *maxlines*, which has a default of 15.  Because a new configuration option is used, there should be no interference between user settings for old releases and future releases. 

Initially, context lines are added to an expanded editor window.  When the height of the window has been maximized, perhaps with the zoomheight feature, and when the height of the window has been reset with the mouse, context lines replace normal code text.

New changeset bed523ba03c4525c9c79a6df700284b6c43024b3 by Terry Jan Reedy in branch 'master':
bpo-33642 and bpo-33679: Revise and condense blurbs (GH-7500)
https://github.com/python/cpython/commit/bed523ba03c4525c9c79a6df700284b6c43024b3

New changeset ab5da50d82766acb49ba162dd61b0f8f19424e29 by Miss Islington (bot) in branch '3.7':
bpo-33642 and bpo-33679: Revise and condense blurbs (GH-7500)
https://github.com/python/cpython/commit/ab5da50d82766acb49ba162dd61b0f8f19424e29

New changeset d071ab12c85636cd15c8ef33647dad9db8441afb by Miss Islington (bot) in branch '3.6':
bpo-33642 and bpo-33679: Revise and condense blurbs (GH-7500)
https://github.com/python/cpython/commit/d071ab12c85636cd15c8ef33647dad9db8441afb