Clamp editor intrinsic height in small viewports

[Michael Greene (mgreenegit)]

Problem

The editor calculates its text area's requested height by subtracting the rows reserved for surrounding UI from the viewport height:

Search state	Reserved rows
Hidden or disabled	2
Search	4
Replace	5

When the viewport is shorter than that reservation, the result is negative. For example, a three-row viewport with Replace active requests a text-area height of 3 - 5 = -2.

The intrinsic-size setter stores that value without clamping it. The layout engine then uses intrinsic heights to calculate parent sizes and position subsequent children. The negative result therefore enters layout arithmetic; it is not merely discarded before layout.

Change

Keep the existing reserved-row calculation, but clamp the resulting text-area height to zero:

text height = max(viewport height - reserved rows, 0)

This preserves the previous result whenever enough space is available. When the reserved rows consume all available height, the text area requests zero rows rather than a negative number.

The calculation is extracted into a private function so its boundary behavior can be tested without initializing a terminal or constructing editor state. The production drawing path calls that same function.

The clamp is local to the code that converts the editor's viewport and UI reservation into a requested text height. It does not change the layout engine's semantics for other callers or redesign the existing row reservations.

Value and scope

This removes a negative-size input from the existing editor layout and gives insufficient-height boundaries an explicit, regression-tested result.

• One source file; no dependencies, public API, settings, commands, or platform-specific code.
• Existing height calculations remain unchanged whenever the reserved rows fit.
• No claim that all controls become visible or usable in an arbitrarily short terminal. The guarantee is limited to a nonnegative requested text-area height.

Validation

The regression covers hidden, disabled, search, and replace states, with:

• Heights 0 through 8, below, at, and above each reservation.
• Decreasing and increasing sequences of those heights.
• Ordinary heights of 24 and 80, and the maximum supported coordinate height.
• Assertions for both nonnegative output and the unchanged result when sufficient space exists.

During preparation, the regression failed before the clamp: height 1 with 2 reserved rows produced -1. The proposed branch preserves that tested patch.

Local verification of this branch on Windows with Rust 1.97.1:

cargo test --offline --locked --package edit --bin edit
# 3 passed; 0 failed; 0 ignored

cargo clippy --offline --locked --package edit --bin edit --tests -- --no-deps --deny warnings
# passed

These are calculation-level tests, not an end-to-end terminal-resize test or evidence of a reproduced crash. Cross-platform validation is left to upstream CI; local results are not a claim that CI has passed.