scenarios:
* Zoom = 1
* pan with mouse: ✓
* pan with up arrow: ✓
* pan with down arrow: ✓
* Zoom < 1
* pan with mouse: ✓
* pan with up arrow: ✓
* pan with down arrow: ✗
* Zoom > 1
* pan with mouse: ✓
* pan with up arrow: ✗
* pan with down arrow: ✓
What ✓ means:
* pan with mouse: lines don't slide relative to the surface
* will still slide relative to the surface when zooming in/out;
that's unavoidable because we want integer pixels for crisp text
* pan with keyboard: at least some part of cursor is always peeking within the viewport
* might still look ugly, with the line containing the cursor almost invisible,
but hitting the down arrow will never pan upwards, or vice versa
Still not working though. I'm pretty much guaranteeing by construction that if
Viewport.y was set from screen_top1, then screen_top1 will not be perturbed.
And yet using scale() inside update_editor_box is incorrect. Hmm..
Well, almost. I'm just reminding myself of the sort of plumbing I need,
not reintroducing the old logic that never worked right and had
undergone n iterations of corruption.
I don't know why this was so hard, but I don't need this variable
preserve_screen_top_of_cursor_node at all. We only set it when the
cursor is in some node, but we also only check for when the current node
is the cursor. Comparing with a nil cursor node works just as well.
I've also checked that driver.love doesn't need
preserve_screen_top_of_cursor_node. I think it came from pensieve.love,
where I've since taken it out. Did I ever need it even there?
scenario:
- create a new definition with ctrl+n
- type in an initial comment
- hit enter
- type in a definition like 'Foo = nil'
- hit F4
- close and restart the driver
Before this commit, the definition no longer showed up on the surface.
(Adding it a second time worked fine.)
I wish I'd included this fix in merge commit 1267010251 back in July.
I created this bug when I implemented GET*, but it was already latent
before that, when I lazily used get_cmd_from_buffer to get the
definition name from buffer instead of creating two different names with
identical definitions.