Fix undefined error in getViewLineMinColumn when viewport has invalid line numbers

[Copilot]

ViewportStart.update() crashes with Cannot read properties of undefined (reading 'getViewLineMinColumn') when called with startLineNumber = 0. This occurs when getLinesViewportData() returns line 0 (e.g., all lines hidden or empty view), causing getIndexOf(-1) to return undefined, leading to modelLineProjections[NaN] access.

Changes

• Added bounds validation in ViewportStart.update() to early-return when startLineNumber < 1 or startLineNumber > lineCount
• Added regression test for setViewport with invalid line numbers

public update(viewModel: IViewModel, startLineNumber: number): void {
    if (startLineNumber < 1 || startLineNumber > viewModel.getLineCount()) {
        return;
    }
    // ...
}

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Unhandled Error] Cannot read properties of undefined (reading 'getViewLineMinColumn')</issue_title>
<issue_description>Issue created from VS Code Errors Analysis Dashboard

Error Bucket

a4930d15-126a-0fda-da7e-400d10c6a1cd

Error Message

Cannot read properties of undefined (reading 'getViewLineMinColumn')

Stack Trace

TypeError: Cannot read properties of undefined (reading 'getViewLineMinColumn')
at Yts.getViewLineMinColumn (file:/Users/cloudtest/vss/_work/1/s/src/vs/editor/common/viewModel/viewModelLines.ts:739:61)
at iis.getLineMinColumn (file:/Users/cloudtest/vss/_work/1/s/src/vs/editor/common/viewModel/viewModelImpl.ts:769:21)
at A5i.update (file:/Users/cloudtest/vss/_work/1/s/src/vs/editor/common/viewModel/viewModelImpl.ts:1289:125)
at iis.setViewport (file:/Users/cloudtest/vss/_work/1/s/src/vs/editor/common/viewModel/viewModelImpl.ts:745:22)
at Object.renderText (file:/Users/cloudtest/vss/_work/1/s/src/vs/editor/browser/view.ts:580:28)
at Object.renderText (file:/Users/cloudtest/vss/_work/1/s/src/vs/editor/browser/view.ts:520:22)
at (file:/Users/cloudtest/vss/_work/1/s/src/vs/editor/browser/view.ts:857:46)
at func (file:/Users/cloudtest/vss/_work/1/s/src/vs/editor/browser/view.ts:790:9)
at safeInvokeNoArg (file:/Users/cloudtest/vss/_work/1/s/src/vs/editor/browser/view.ts:857:14)
at $2e.t [as g] (file:/Users/cloudtest/vss/_work/1/s/src/vs/editor/browser/view.ts:840:9)

Details

Property	Value
Version	1.107.1
Commit	994fd12f
Last Seen	2025-12-18T23:59:48.224Z
Total Hits	2.2M
Affected Users	52.1K
Platforms	Linux, Mac, Windows
Product	VSCode

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This issue was automatically created from the VS Code Errors Dashboard</issue_description>

<agent_instructions>analyze the unhandled error, based on the callstack and its deminified source (with the commit hash), figure out how or what scenarios may trigger the error, propose possible fixes, and then implement the best option to fix it.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes [Unhandled Error] Cannot read properties of undefined (reading 'getViewLineMinColumn') #284539

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