I can't believe I didn't catch this until now. All I had to do is open
MobyDick.markdown from https://www.hogbaysoftware.com/posts/moby-dick-workout,
press page-down and click on the top screen line (or any screen line
containing the same line as the top screen line). Easy to catch with any
file containing lots of overly long lines, as happens in particular at
the start of Moby Dick.
I _had_ seen this problem before, but it seemed to disappear after
unrelated changes, and I convinced myself I'd fixed it as a side-effect.
The bug just failed to manifest if the top line happened to start at the
top of the screen. Scroll down a few pages in Moby Dick and the dialogue
starts and line length drops precipitously.
Scenario: backspacing until a line takes up fewer screen lines, then
pressing `down`.
I've gone through and checked that fragments and screen_line_starting_pos
are now cleared together everywhere.
I'm giving up finding a more generalized solution. The issue is that we
need the correct selection state right up to the point where we modify
Lines, in order to capture precise undo state.
Hopefully there aren't any other keychords that should clear the
selection.
We just need to ensure textinput events never make use of selection
state.
All tests are passing, but I'm aware of a couple of issues. But now we
can keep all the special cases in one place.
Now all the cases that clear Selection1 do so in a very consistent way
at the end of each case. And cases that set Selection1 symmetrically do
so at the start of each case.
Bugfix: we want selections to persist even when we lift up the shift
key.
This requires hoisting some code inside every case inside the whole
keypress hierarchy, to ensure we never clear selections before
textinput events can handle them.
Current cross-cutting concerns we're explicitly scattering code for.
- autosave
- undo
- selection management
To reproduce:
click to position cursor at end of a line
hit enter
press any key
before:
newline got erased and key got added to previous line
now:
newline is preserved
The new test checks a generalization of this.
It's important that the error be additive rather than multiplicative,
otherwise the area grows asymmetrically along a line.
Hopefully freehand drawings will work more intuitively now.