fix(editor): hide an emphasis whose text has been deleted instead of drawing it somewhere else

[datlechin]

Fixes #2341.

The problem

EmphasisManager.updateLayerBackgrounds() runs on every draw and skips any emphasis whose shape it cannot build. Skipping leaves that emphasis's CAShapeLayer holding the last path it was given, so once the text under it is edited away the highlight goes on painting at its old position, over whatever now occupies that place.

Search highlights had a worse version of it. roundedPathForRange answers a range whose length no longer fits with rectForEndOffset(), the caret rect at the end of the document, so a stale .standard emphasis did not keep its old position: it jumped to the end of the editor and drew a highlight over nothing.

This was split out of #2338, which fixed the crash on the same code path and deliberately left the behaviour alone, and the risk it names is why: an attribute-only edit invalidates lines, so clearing too eagerly could make a find highlight blink off and on during ordinary syntax highlighting traffic.

The fix

Two parts, and the second is what makes the first safe.

• makeShapePath returns nil when the emphasis range no longer fits the document, so a range that outran the text stops resolving to the caret rect at the end. An emphasis marks specific text; once that text is gone there is no shape to draw.
• The skip path hides the emphasis and text layers instead of leaving them visible with a stale path, and the draw path unhides them.

Hiding rather than clearing the path is what answers the flicker risk. A line that is invalidated and re-typeset inside the same layout pass has its rects back before the draw, so nothing changes for it. A line invalidated outside the layout window has no rects and is not on screen either, so hiding it is not observable. When the range lays out again the layer is unhidden with its shape intact, rather than having to be rebuilt.

Verification

• swift test --package-path LocalPackages/CodeEditTextView: 167 tests in 16 suites pass, including 2 new ones. Against the unfixed code anEmphasisWhoseTextIsGoneStopsBeingDrawn fails: the layer is still visible after its text is deleted.
• The control case, anEmphasisWhoseTextIsStillThereKeepsBeingDrawn, fails any fix that hides too much.
• xcodebuild -scheme TablePro build: passes.

No UI automation: the assertion is about which layers are hidden inside the editor's own layer tree, which XCUITest cannot see.

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