Text before the caret: no UIA TextPattern on classic Win32 edit controls, fall back to ValuePattern and EM_GETSEL

Goal

Before pasting dictated text, the app wants up to 160 characters that precede the caret in the focused control, so it can continue an unfinished sentence (lower-case start, comma where needed). The focused control is found with UI Automation (IUIAutomation.GetFocusedElement).

TextPattern path

For modern controls the app asks the element for UIA_TextPatternId, then:

• GetSelection(); no selection ranges means "unknown".
• The caret range is the selection's first range. Only an editable field counts: GetAttributeValue(UIA_IsReadOnlyAttributeId) must be exactly False. A web page or document that is being read also reports a caret, and its text is not wanted.
• A cloned range is moved so that it ends at the start of the caret and starts chars characters earlier (MoveEndpointByRange, MoveEndpointByUnit with a negative count).
• The text is read from that range and CRLF is normalised.

Clamp to the field in browsers

In a browser, moving the start endpoint back carries on past the beginning of the field into the page around it. A dictation into an empty box then started with ", " because the page text before the field was picked up as "text before the caret". The fix (changelog, 2026-11-13) compares the range start with the field's DocumentRange start and moves it back to the field start when it lies before it; if the clamped range is empty the result is an empty string, meaning "start of the field".

Classic edit controls have no TextPattern

When GetCurrentPattern(UIA_TextPatternId) returns nothing, the element is often a classic Win32 Edit control (WinForms text boxes are an example the author tested). Those controls expose ValuePattern (the whole text) but no caret position through UI Automation. The fallback:

• Require a native window handle (CurrentNativeWindowHandle), a ValuePattern, and a class name containing "edit".
• Skip read-only controls (CurrentIsReadOnly).
• Send EM_GETSEL (0x00B0) with SendMessageTimeoutW, flag SMTO_ABORTIFHUNG and a 200 ms timeout, so a hung target cannot block the dictation. The selection start is in the low word of the result.
• If the low word is 0xFFFF the position is treated as unknown (the code comment says it is beyond what this form of the message reports), otherwise the text is value[:start][-chars:].

EM_GETSEL is a message to the control, so it works only for controls that implement it; custom-drawn or rich controls that expose neither pattern simply return "unknown" and the dictation is pasted without continuation logic.

Related reading-side behaviour

• A separate "read the whole field" helper tries TextPattern.DocumentRange.GetText(max) first and ValuePattern.CurrentValue second, and gives up on fields of 200,000 characters or more.
• Terminals and some custom controls expose neither pattern; the helper returns None rather than guessing.

Unverified

• Only a WinForms text box was verified directly for the fallback. The notes list Outlook, Teams and other browsers as untested at the time of the first version, and a browser case was fixed afterwards.
• The statement that "classic Win32 edit boxes have no TextPattern" is the author's observation on the tested control, not a survey of every edit-control variant (for example rich edit controls).