debug: preserve the top of stack when stepping in or at BP (microsoft#214902)

Refs  microsoft#214433

Author: connor4312

src/vs/workbench/contrib/debug/common/debugModel.ts

@@ -540,6 +540,8 @@ export class StackFrame implements IStackFrame {
 	}
 }
 
+const KEEP_SUBTLE_FRAME_AT_TOP_REASONS: readonly string[] = ['breakpoint', 'step', 'function breakpoint'];
+
 export class Thread implements IThread {
 	private callStack: IStackFrame[];
 	private staleCallStack: IStackFrame[];
@@ -578,10 +580,11 @@ export class Thread implements IThread {
 
 	getTopStackFrame(): IStackFrame | undefined {
 		const callStack = this.getCallStack();
+		const stopReason = this.stoppedDetails?.reason;
 		// Allow stack frame without source and with instructionReferencePointer as top stack frame when using disassembly view.
 		const firstAvailableStackFrame = callStack.find(sf => !!(
-			((this.stoppedDetails?.reason === 'instruction breakpoint' || (this.stoppedDetails?.reason === 'step' && this.lastSteppingGranularity === 'instruction')) && sf.instructionPointerReference) ||
-			(sf.source && sf.source.available && !isFrameDeemphasized(sf))));
+			((stopReason === 'instruction breakpoint' || (stopReason === 'step' && this.lastSteppingGranularity === 'instruction')) && sf.instructionPointerReference) ||
+			(sf.source && sf.source.available && (KEEP_SUBTLE_FRAME_AT_TOP_REASONS.includes(stopReason!) || !isFrameDeemphasized(sf)))));
 		return firstAvailableStackFrame;
 	}