chat: a click away from selected text clears it

[foxnne]

A selection in the transcript stayed when the person clicked empty space, another message or the message box, so several messages could show a selection at once and none went away. Now a selection goes when the person presses anywhere outside the text it is in: another message, empty space, the message box, elsewhere in fizzy. Copy (⌘C) of a selection still works.

How

dvui keeps a TextLayoutWidget's selection after it loses focus. The chat tracks which widget inside the transcript holds focus (dvui.lastFocusedIdInFrameSince around the scroll area: a text the person pressed in, or the scroll area after a click on empty space). When that changes, it tells the old text to select nothing with Window.addEventTextSelect(.{ .start = 0, .end = 0, .target_id = old }) at the top of the next frame. That is the plugin's beginFrame, before any widget runs, which is when dvui asks for added events. A text given a selection scrolls its cursor into view, so on that frame the chat puts the viewport back where it set it. drawTranscript's height and scroll code is otherwise unchanged: one call before the scroll area, one after.

Selection across messages: what's missing

A selection that spans several messages, the way a document's does, isn't possible from the plugin:

• dvui: a selection belongs to one TextLayoutWidget, as byte offsets into its own text, and a drag is captured by the layout it started in. Nothing lets a drag continue into a sibling layout, and nothing asks a layout which byte is under a point without capturing (findPoint is private). A multi-layout selection needs a selection owned above the layouts (anchor and focus as layout + byte), handed to each layout through InitOptions.selection (already public), plus a public point-to-byte query and a way to keep a drag going across layouts.
• markdown service: even with that, a message's text is laid out by the markdown plugin (markdown.Api.renderInline), one layout per paragraph, and InlineOptions has only width, image_base_dir and id_extra. The service can't take a selection, report the selected text, or clear it. The seam is an InlineOptions field for a caller-owned selection (or selectedText(id_extra) / clearSelection(id_extra) on the service). That's an SDK change in fizzy, and once it exists this PR's focus tracking becomes a direct clear.

Within one paragraph, selecting, double/triple-click and copy are dvui's, as before.

Seen

macOS, in a sandbox profile with a seeded sample transcript, driven by a demo tape (no OS events), with captures before and after:

• Before: text selected in one message stays when a second message's text is selected (both highlighted), and stays after a click on empty space.
• After: selecting in the second message clears the first. A click on empty space clears the second. A click in the message box clears it too. ⌘C, then ⌘V into the message box, pastes the selected words.
• I couldn't scroll the transcript from a tape (wheel ticks didn't move it), so the "keeps the viewport" path, where a selection sits half off screen when cleared, wasn't seen.

zig fmt --check src plugin.zig clean; built against the fizzy main SDK (--fork).

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