Explore output with the keyboard: Enter/Escape, siblings, Shift+arrows (#2382)

[gunterkoenigsmann]

Requested by Gunter · project thread

Steps two and three of #2382. Step one was #2383 and #2384.

Before: the keyboard could select a whole result of a cell's output, but nothing smaller. Enter on a selected result opened a new input cell holding its text.

After: with a part of an output selected, Enter goes into it and selects its first part: from a result to its expression, then from a fraction to its numerator, from a function to its name, from a matrix to its first entry, and so on. Escape goes back out one level at a time, and from a whole result back into the cell's input. Left and Right select the neighbouring part. In a matrix, all four arrow keys move from entry to entry and skip elided ones. Shift+Left and Shift+Right grow or shrink the selection over neighbouring parts. A matrix entry is selected as a one-entry block, so Shift+arrows grow it the same way they grow a block dragged with the mouse (#2370). Every move raises the same accessibility focus event step one does.

A few choices worth checking:

• Enter on a part that has no parts (a lone x) keeps its old meaning: it opens a new cell holding the selection. So the old Enter still works on leaves, but not on whole results.
• Up and Down below the result level keep doing what they did for a mouse-selected part: they leave the cell. Step one's tests pin that down, so I left it alone.
• Left and Right on a whole result are unchanged, so they still step through an animation's frames.
• Shift+Left/Right can't span numerator and denominator. Those are two separate cell lists, and a selection is a run within one list, so the key does nothing there.

How: a new OutputNavigation (in src/worksheet/) models the output as a tree built from the expression's structure (OnInner()), not from the draw list, so a line break doesn't change what a key does. Glyphs that a cell only draws while broken into lines (sqrt(, ,, ), /, ^, a limit's lim) are left out, and so are hidden and empty cells and elided matrix entries. An item that only wraps a single part, such as a matrix entry holding one fraction, is looked through, so that Enter always visibly changes the selection. It knows nothing about key events. Worksheet::NavigateOutput() maps the keys onto it. The node is found by searching the tree, so no state goes stale when the mouse changes the selection. The one exception is the Shift anchor, which is a CellPtr.

Tests: test_OutputKeyboardNavigation gains 5 scenarios, covering Enter/Escape round trips, glyph filtering, Enter on a leaf, Shift growing and shrinking from either end, and matrix entry movement, block growth and entering an entry. With the key handler disabled, 16 of the new assertions fail. All 58 unit tests pass locally with a unity build. I also checked it live in Xvfb on matrix([1,x/(y+1)],[3,sin(z)]) and a+b^2+sqrt(c): the entry and the numerator highlight, and so does a run grown with Shift. That live check is what found the numerator/denominator case, which now has its own test.

Docs: a new manual section, "Exploring output with the keyboard" (it covers step one too), and a NEWS entry.

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